diff --git a/valohai/config.py b/valohai/config.py
index 42709c7..d5e8117 100644
--- a/valohai/config.py
+++ b/valohai/config.py
@@ -1,5 +1,5 @@
 import os
 
 
-def is_running_in_valohai():
+def is_running_in_valohai() -> bool:
     return bool(os.environ.get("VH_JOB_ID"))
diff --git a/valohai/inputs.py b/valohai/inputs.py
index 9e9ac62..65a4f92 100644
--- a/valohai/inputs.py
+++ b/valohai/inputs.py
@@ -7,7 +7,7 @@
 
 
 class Input:
-    def __init__(self, name: str):
+    def __init__(self, name: str) -> None:
         self.name = str(name)
 
     def paths(
diff --git a/valohai/internals/compression.py b/valohai/internals/compression.py
index 37c1e9e..a643907 100644
--- a/valohai/internals/compression.py
+++ b/valohai/internals/compression.py
@@ -60,12 +60,18 @@ def put(self, archive_name, source: Union[str, IO]):
 
 
 class ZipArchive(BaseArchive, zipfile.ZipFile):
-    def __init__(self, file, mode="r", *, compresslevel=1):
+    def __init__(self, file: str, mode: str = "r", *, compresslevel: int = 1) -> None:
         # Only Python 3.7+ has the compresslevel kwarg here
         super().__init__(file, mode, compression=zipfile.ZIP_STORED)
         self.compresslevel = compresslevel
 
-    def writestream(self, arcname, data, compress_type, compresslevel):
+    def writestream(
+        self,
+        arcname: str,
+        data: Union[str, bytes, IO[bytes]],
+        compress_type: int,
+        compresslevel: int,
+    ) -> None:
         # Like `writestr`, but also supports a stream (and doesn't support directories).
         zinfo = zipfile.ZipInfo(filename=arcname)
         zinfo.compress_type = compress_type
@@ -82,7 +88,7 @@ def writestream(self, arcname, data, compress_type, compresslevel):
                 shutil.copyfileobj(data, dest, 524288)
         assert zinfo.file_size
 
-    def put(self, archive_name, source: Union[str, IO]):
+    def put(self, archive_name: str, source: Union[str, IO]) -> None:
         compress_type = (
             zipfile.ZIP_DEFLATED
             if guess_compressible(archive_name)
@@ -103,7 +109,7 @@ def put(self, archive_name, source: Union[str, IO]):
 
 
 class TarArchive(BaseArchive, tarfile.TarFile):
-    def put(self, archive_name, source: Union[str, IO]):
+    def put(self, archive_name: str, source: Union[str, IO]) -> None:
         with contextlib.ExitStack() as es:
             if isinstance(source, str):
                 size = os.stat(source).st_size
@@ -119,7 +125,7 @@ def put(self, archive_name, source: Union[str, IO]):
             self.addfile(tarinfo, stream)
 
 
-def open_archive(path: str):
+def open_archive(path: str) -> BaseArchive:
     if path.endswith(".zip"):
         return ZipArchive(path, "w")
     elif path.endswith(".tar"):
diff --git a/valohai/internals/download.py b/valohai/internals/download.py
index 9502a2c..f1f5937 100644
--- a/valohai/internals/download.py
+++ b/valohai/internals/download.py
@@ -3,7 +3,7 @@
 
 
 # TODO: This is close to valohai-local-run. Possibility to merge.
-def download_url(url, path, force_download=False):
+def download_url(url: str, path: str, force_download: bool = False) -> str:
     if not os.path.isfile(path) or force_download:
         try:
             import requests
diff --git a/valohai/internals/files.py b/valohai/internals/files.py
index 7048f86..d07183f 100644
--- a/valohai/internals/files.py
+++ b/valohai/internals/files.py
@@ -5,7 +5,7 @@
 from typing import Set, Union
 
 
-def set_file_read_only(path: str):
+def set_file_read_only(path: str) -> None:
     os.chmod(path, S_IREAD | S_IRGRP | S_IROTH)
 
 
diff --git a/valohai/internals/guid.py b/valohai/internals/guid.py
index 2938ea7..21d6b59 100644
--- a/valohai/internals/guid.py
+++ b/valohai/internals/guid.py
@@ -4,7 +4,7 @@
 _execution_guid = None
 
 
-def get_execution_guid():
+def get_execution_guid() -> str:
     global _execution_guid
     if not _execution_guid:
         _execution_guid = f'{time.strftime("%Y%m%d-%H%M%S")}-{uuid.uuid4().hex[:6]}'
diff --git a/valohai/internals/input_info.py b/valohai/internals/input_info.py
index 7086b4e..e5cf35d 100644
--- a/valohai/internals/input_info.py
+++ b/valohai/internals/input_info.py
@@ -10,17 +10,25 @@
 
 
 class FileInfo:
-    def __init__(self, *, name, uri, path, size, checksums):
+    def __init__(
+        self,
+        *,
+        name: str,
+        uri: Optional[str],
+        path: Optional[str],
+        size: Optional[int],
+        checksums: Optional[dict],
+    ) -> None:
         self.name = str(name)
         self.uri = str(uri) if uri else None
         self.checksums = checksums
         self.path = str(path) if path else None
         self.size = int(size) if size else None
 
-    def is_downloaded(self):
+    def is_downloaded(self) -> Optional[bool]:
         return self.path and os.path.isfile(self.path)
 
-    def download(self, path, force_download: bool = False):
+    def download(self, path: str, force_download: bool = False) -> None:
         self.path = download_url(
             self.uri, os.path.join(path, self.name), force_download
         )
@@ -31,13 +39,13 @@ class InputInfo:
     def __init__(self, files: Iterable[FileInfo]):
         self.files = list(files)
 
-    def is_downloaded(self):
+    def is_downloaded(self) -> bool:
         if not self.files:
             return False
 
         return all(f.is_downloaded() for f in self.files)
 
-    def download(self, path, force_download: bool = False):
+    def download(self, path: str, force_download: bool = False) -> None:
         for f in self.files:
             f.download(path, force_download)
 
diff --git a/valohai/internals/merge.py b/valohai/internals/merge.py
index 7259190..d1ddddf 100644
--- a/valohai/internals/merge.py
+++ b/valohai/internals/merge.py
@@ -1,7 +1,8 @@
 import copy
 
-from valohai_yaml.objs import Config, Step
 from valohai_yaml.objs.base import Item
+from valohai_yaml.objs.config import Config
+from valohai_yaml.objs.step import Step
 from valohai_yaml.utils.merge import merge_dicts, merge_simple
 
 from valohai.consts import DEFAULT_DOCKER_IMAGE
diff --git a/valohai/internals/parsing.py b/valohai/internals/parsing.py
index 0d44d63..35e6542 100644
--- a/valohai/internals/parsing.py
+++ b/valohai/internals/parsing.py
@@ -3,7 +3,7 @@
 from typing import List
 
 
-def is_module_function_call(node, module, function):
+def is_module_function_call(node: ast.Call, module: str, function: str) -> bool:
     try:
         return node.func.attr == function and node.func.value.id == module
     except AttributeError:
@@ -39,25 +39,25 @@ class PrepareParser(ast.NodeVisitor):
         valohai.prepare(parameters=get_parameters())
     """
 
-    def __init__(self):
+    def __init__(self) -> None:
         self.assignments = {}
         self.parameters = {}
         self.inputs = {}
         self.step = None
         self.image = None
 
-    def visit_Assign(self, node):
+    def visit_Assign(self, node: ast.Assign) -> None:
         try:
             self.assignments[node.targets[0].id] = ast.literal_eval(node.value)
         except ValueError:
             # We don't care about assignments that can't be literal_eval():ed
             pass
 
-    def visit_Call(self, node):
+    def visit_Call(self, node: ast.Call) -> None:
         if is_module_function_call(node, "valohai", "prepare"):
             self.process_valohai_prepare_call(node)
 
-    def process_valohai_prepare_call(self, node):
+    def process_valohai_prepare_call(self, node: ast.Call) -> None:
         self.step = "default"
         if hasattr(node, "keywords"):
             for key in node.keywords:
@@ -70,7 +70,7 @@ def process_valohai_prepare_call(self, node):
                 elif key.arg == "image":
                     self.image = ast.literal_eval(key.value)
 
-    def process_default_inputs_arg(self, key):
+    def process_default_inputs_arg(self, key: ast.keyword) -> None:
         if isinstance(key.value, ast.Name) and key.value.id in self.assignments:
             self.inputs = {
                 key: value if isinstance(value, List) else [value]
@@ -84,7 +84,7 @@ def process_default_inputs_arg(self, key):
         else:
             raise NotImplementedError()
 
-    def process_default_parameters_arg(self, key):
+    def process_default_parameters_arg(self, key: ast.keyword) -> None:
         if isinstance(key.value, ast.Name) and key.value.id in self.assignments:
             self.parameters = self.assignments[key.value.id]
         elif isinstance(key.value, ast.Dict):
diff --git a/valohai/internals/vfs.py b/valohai/internals/vfs.py
index 0ee0951..3b30ad7 100644
--- a/valohai/internals/vfs.py
+++ b/valohai/internals/vfs.py
@@ -3,9 +3,9 @@
 import shutil
 import tempfile
 from contextlib import ExitStack
-from tarfile import TarFile, TarInfo
+from tarfile import ExFileObject, TarFile, TarInfo
 from typing import IO, Optional, Union
-from zipfile import ZipFile, ZipInfo
+from zipfile import ZipExtFile, ZipFile, ZipInfo
 
 
 class File:
@@ -14,7 +14,7 @@ class File:
     def open(self) -> io.BufferedReader:
         raise NotImplementedError("...")
 
-    def read(self):
+    def read(self) -> bytes:
         with self.open() as f:
             return f.read()
 
@@ -35,12 +35,14 @@ class FileOnDisk(File):
     path: str
     dir_entry: Optional[os.DirEntry]
 
-    def __init__(self, name, path, dir_entry=None):
+    def __init__(
+        self, name: str, path: str, dir_entry: Optional[os.DirEntry] = None
+    ) -> None:
         self._name = name
         self.path = path
         self.dir_entry = dir_entry
 
-    def open(self):
+    def open(self) -> io.BufferedReader:
         return open(self.path, "rb")  # noqa: SIM115
 
     @property
@@ -51,7 +53,7 @@ def name(self) -> str:
 class FileInContainer(File):
     _concrete_path: Optional[str] = None
 
-    def open_concrete(self, delete=True):
+    def open_concrete(self, delete: bool = True) -> IO[bytes]:
         if self._concrete_path and os.path.isfile(self._concrete_path):
             return open(self._concrete_path)  # noqa: SIM115
         tf = tempfile.NamedTemporaryFile(suffix=self.extension, delete=delete)
@@ -60,7 +62,7 @@ def open_concrete(self, delete=True):
         self._concrete_path = tf.name
         return tf
 
-    def extract(self, destination: Union[str, IO]):
+    def extract(self, destination: Union[str, IO]) -> None:
         if isinstance(destination, str):
             destination = open(destination, "wb")  # noqa: SIM115
             should_close = True
@@ -72,7 +74,7 @@ def extract(self, destination: Union[str, IO]):
             if should_close:
                 destination.close()
 
-    def _do_extract(self, destination: IO):
+    def _do_extract(self, destination: IO) -> None:
         # if a file has a better idea how to write itself into an IO, this is the place
         with self.open() as f:
             shutil.copyfileobj(f, destination)
@@ -82,12 +84,14 @@ class FileInZip(FileInContainer):
     zipfile: ZipFile
     zipinfo: ZipInfo
 
-    def __init__(self, parent_file, zipfile, zipinfo):
+    def __init__(
+        self, parent_file: FileOnDisk, zipfile: ZipFile, zipinfo: ZipInfo
+    ) -> None:
         self.parent_file = parent_file
         self.zipfile = zipfile
         self.zipinfo = zipinfo
 
-    def open(self):
+    def open(self) -> ZipExtFile:
         return self.zipfile.open(self.zipinfo, "r")
 
     @property
@@ -107,12 +111,14 @@ class FileInTar(FileInContainer):
     tarfile: TarFile
     tarinfo: TarInfo
 
-    def __init__(self, parent_file, tarfile, tarinfo):
+    def __init__(
+        self, parent_file: FileOnDisk, tarfile: TarFile, tarinfo: TarInfo
+    ) -> None:
         self.parent_file = parent_file
         self.tarfile = tarfile
         self.tarinfo = tarinfo
 
-    def open(self):
+    def open(self) -> ExFileObject:
         return self.tarfile.extractfile(self.tarinfo)
 
     @property
@@ -144,14 +150,14 @@ def find_files_in_tar(vr: "VFS", df: FileOnDisk) -> None:
 
 
 class VFS:
-    def __init__(self):
+    def __init__(self) -> None:
         self.files = []
         self.exit_stack = ExitStack()
 
-    def __enter__(self):
+    def __enter__(self) -> "VFS":
         return self
 
-    def __exit__(self, *exc_details):
+    def __exit__(self, *exc_details) -> None:
         self.exit_stack.__exit__(*exc_details)
 
 
@@ -160,8 +166,8 @@ def add_disk_file(
     name: str,
     path: str,
     dir_entry: Optional[os.DirEntry] = None,
-    process_archives=False,
-):
+    process_archives: bool = False,
+) -> None:
     disk_file = FileOnDisk(name=name, path=path, dir_entry=dir_entry)
     if process_archives:
         extension = disk_file.extension.lower()
@@ -174,7 +180,7 @@ def add_disk_file(
     vfs.files.append(disk_file)
 
 
-def find_files(vfs: VFS, root: str, *, process_archives: bool):
+def find_files(vfs: VFS, root: str, *, process_archives: bool) -> None:
     dent: os.DirEntry
 
     def _walk(path):
diff --git a/valohai/internals/yaml.py b/valohai/internals/yaml.py
index ef9f318..0164e4c 100644
--- a/valohai/internals/yaml.py
+++ b/valohai/internals/yaml.py
@@ -1,8 +1,10 @@
 import os
 from typing import Any, Dict
 
-from valohai_yaml.objs import Config, Parameter, Step
+from valohai_yaml.objs.config import Config
 from valohai_yaml.objs.input import Input, KeepDirectories
+from valohai_yaml.objs.parameter import Parameter
+from valohai_yaml.objs.step import Step
 
 from valohai.consts import DEFAULT_DOCKER_IMAGE
 from valohai.internals.parsing import parse
@@ -17,7 +19,7 @@ def generate_step(
     step: str,
     image: str,
     parameters: ParameterDict,
-    inputs: InputDict
+    inputs: InputDict,
 ) -> Step:
     config_step = Step(
         name=step,
@@ -50,7 +52,7 @@ def generate_config(
     step: str,
     image: str,
     parameters: ParameterDict,
-    inputs: InputDict
+    inputs: InputDict,
 ) -> Config:
     step = generate_step(
         relative_source_path=relative_source_path,
@@ -85,7 +87,7 @@ def get_source_relative_path(source_path: str, config_path: str) -> str:
     return os.path.join(relative_source_dir, os.path.basename(source_path))
 
 
-def parse_config_from_source(source_path: str, config_path: str):
+def parse_config_from_source(source_path: str, config_path: str) -> Config:
     with open(source_path) as source_file:
         parsed = parse(source_file.read())
         if not parsed.step:
diff --git a/valohai/metadata.py b/valohai/metadata.py
index db35944..8143010 100644
--- a/valohai/metadata.py
+++ b/valohai/metadata.py
@@ -1,20 +1,21 @@
 import json
+from typing import Any
 
 _supported_types = [int, float]
 
 
 class Logger:
-    def __init__(self):
+    def __init__(self) -> None:
         self.partial_logs = {}
 
-    def __enter__(self):
+    def __enter__(self) -> "Logger":
         self.partial_logs = {}
         return self
 
-    def __exit__(self, type, value, traceback):
+    def __exit__(self, type, value, traceback) -> None:
         self.flush()
 
-    def log(self, *args, **kwargs):
+    def log(self, *args, **kwargs) -> None:
         """Log a single name/value pair to be flushed into standard output later as batch.
 
         For a repeating iteration like a machine learning training loop, Valohai expects
@@ -54,7 +55,7 @@ def log(self, *args, **kwargs):
         for key, value in kwargs.items():
             self._serialize(key, value)
 
-    def flush(self):
+    def flush(self) -> None:
         """Flush all the partial logs into standard as a batch.
 
         For a repeating iteration like a machine learning training loop, Valohai expects
@@ -74,7 +75,7 @@ def flush(self):
             print(f"\n{json.dumps(self.partial_logs, default=str)}")  # noqa
             self.partial_logs.clear()
 
-    def _serialize(self, name, value):
+    def _serialize(self, name: str, value: Any) -> None:
         self.partial_logs.update({str(name): value})
 
 
diff --git a/valohai/outputs.py b/valohai/outputs.py
index 9ec13dd..f662447 100644
--- a/valohai/outputs.py
+++ b/valohai/outputs.py
@@ -14,7 +14,7 @@
 
 
 class Output:
-    def __init__(self, name: str = ""):
+    def __init__(self, name: str = "") -> None:
         self.name = str(name)
 
     def path(self, filename: str, makedirs: bool = True) -> str:
@@ -52,7 +52,7 @@ def path(self, filename: str, makedirs: bool = True) -> str:
 
         return path
 
-    def live_upload(self, filename: str):
+    def live_upload(self, filename: str) -> None:
         for file_path in glob.glob(get_glob_pattern(self.path(filename))):
             set_file_read_only(file_path)
 
diff --git a/valohai/parameters.py b/valohai/parameters.py
index 690e199..7c1be5b 100644
--- a/valohai/parameters.py
+++ b/valohai/parameters.py
@@ -1,10 +1,9 @@
 from valohai.internals import global_state
-
-from .internals.parameters import load_parameter, supported_types
+from valohai.internals.parameters import load_parameter, supported_types
 
 
 class Parameter:
-    def __init__(self, name: str, default: supported_types = None):
+    def __init__(self, name: str, default: supported_types = None) -> None:
         self.name = str(name)
         self.default = default
 
diff --git a/valohai/utils.py b/valohai/utils.py
index 68fe88d..22b8971 100644
--- a/valohai/utils.py
+++ b/valohai/utils.py
@@ -16,7 +16,7 @@ def prepare(
     default_parameters: Optional[dict] = None,
     default_inputs: Optional[dict] = None,
     image: str = None,
-):
+) -> None:
     """Define the name of the step and it's required inputs, parameters and Docker image
 
     Has dual purpose:
diff --git a/valohai/yaml.py b/valohai/yaml.py
index 9382c33..e6a8ef8 100644
--- a/valohai/yaml.py
+++ b/valohai/yaml.py
@@ -1,7 +1,7 @@
 from collections import OrderedDict
 
 import yaml
-from valohai_yaml.objs import Config
+from valohai_yaml.objs.config import Config
 
 # https://stackoverflow.com/questions/42518067/how-to-use-ordereddict-as-an-input-in-yaml-dump-or-yaml-safe-dump
 yaml.add_representer(
@@ -12,7 +12,7 @@
 )
 
 
-def config_to_yaml(config: Config):
+def config_to_yaml(config: Config) -> str:
     """Serialize Valohai Config to YAML
 
     :param config: valohai_yaml.objs.Config object