diff --git a/README.md b/README.md
index 45a0a67..ddc3e55 100644
--- a/README.md
+++ b/README.md
@@ -35,6 +35,7 @@ A simple utility to quickly replace text in one or more files or globs. Works sy
   - [Making replacements on network drives](#making-replacements-on-network-drives)
   - [Specify character encoding](#specify-character-encoding)
   - [Dry run](#dry-run)
+  - [File system](#file-system)
 - [CLI usage](#cli-usage)
 - [A note on using globs with the CLI](#a-note-on-using-globs-with-the-cli)
 - [Version information](#version-information)
@@ -414,6 +415,32 @@ const options = {
 };
 ```
 
+### File system
+`replace-in-file` defaults to using `require('fs')` to provide file reading and write APIs.
+You can provide an `fs` object of your own to switch to a different file system, such as a mock file system for unit tests.
+
+* If using asynchronous APIs, the provided `fs` must provide `readFile` and `writeFile` methods
+* If using synchronous APIs, the provided `fs` must provide `readFileSync` and `writeFileSync` methods
+
+Custom `fs` methods should have the same parameters and returned values as their [built-in Node `fs`](https://nodejs.org/api/fs.html) equivalents.
+
+```js
+replaceInFile({
+  from: 'a',
+  fs: {
+    readFile: (file, encoding, callback) => {
+      console.log(`Reading ${file} with encoding ${encoding}...`);
+      callback(null, 'fake file contents');
+    },
+    writeFile: (file, newContents, encoding, callback) => {
+      console.log(`Writing ${file} with encoding ${encoding}: ${newContents}`);
+      callback(null);
+    },
+  },
+  to: 'b',
+})
+```
+
 ## CLI usage
 
 ```sh
diff --git a/lib/helpers/parse-config.js b/lib/helpers/parse-config.js
index 3314624..1223ad4 100644
--- a/lib/helpers/parse-config.js
+++ b/lib/helpers/parse-config.js
@@ -14,6 +14,7 @@ const defaults = {
   dry: false,
   glob: {},
   cwd: null,
+  fs: require('fs'),
 };
 
 /**
diff --git a/lib/helpers/process-async.js b/lib/helpers/process-async.js
index 05cd2f6..9571a62 100644
--- a/lib/helpers/process-async.js
+++ b/lib/helpers/process-async.js
@@ -1,5 +1,4 @@
 
-const fs = require('fs');
 const runProcessors = require('./run-processors');
 
 /**
@@ -8,7 +7,7 @@ const runProcessors = require('./run-processors');
 module.exports = function processAsync(file, processor, config) {
 
   //Extract relevant config
-  const {encoding, dry} = config;
+  const {encoding, dry, fs} = config;
 
   //Wrap in promise
   return new Promise((resolve, reject) => {
diff --git a/lib/helpers/process-sync.js b/lib/helpers/process-sync.js
index fb2b157..34ece12 100644
--- a/lib/helpers/process-sync.js
+++ b/lib/helpers/process-sync.js
@@ -1,5 +1,4 @@
 
-const fs = require('fs');
 const runProcessors = require('./run-processors');
 
 /**
@@ -8,7 +7,7 @@ const runProcessors = require('./run-processors');
 module.exports = function processSync(file, processor, config) {
 
   //Extract relevant config and read file contents
-  const {encoding, dry} = config;
+  const {encoding, dry, fs} = config;
   const contents = fs.readFileSync(file, encoding);
 
   //Process contents and check if anything changed
diff --git a/lib/helpers/replace-async.js b/lib/helpers/replace-async.js
index 2a7aea9..2d22234 100644
--- a/lib/helpers/replace-async.js
+++ b/lib/helpers/replace-async.js
@@ -1,5 +1,4 @@
 
-const fs = require('fs');
 const makeReplacements = require('./make-replacements');
 
 /**
@@ -8,7 +7,7 @@ const makeReplacements = require('./make-replacements');
 module.exports = function replaceAsync(file, from, to, config) {
 
   //Extract relevant config
-  const {encoding, dry, countMatches} = config;
+  const {encoding, dry, countMatches, fs} = config;
 
   //Wrap in promise
   return new Promise((resolve, reject) => {
diff --git a/lib/helpers/replace-sync.js b/lib/helpers/replace-sync.js
index 1d66ae0..2ed1f78 100644
--- a/lib/helpers/replace-sync.js
+++ b/lib/helpers/replace-sync.js
@@ -1,5 +1,4 @@
 
-const fs = require('fs');
 const makeReplacements = require('./make-replacements');
 
 /**
@@ -8,7 +7,7 @@ const makeReplacements = require('./make-replacements');
 module.exports = function replaceSync(file, from, to, config) {
 
   //Extract relevant config and read file contents
-  const {encoding, dry, countMatches} = config;
+  const {encoding, dry, countMatches, fs} = config;
   const contents = fs.readFileSync(file, encoding);
 
   //Replace contents and check if anything changed
diff --git a/lib/process-file.spec.js b/lib/process-file.spec.js
index 0add917..99505db 100644
--- a/lib/process-file.spec.js
+++ b/lib/process-file.spec.js
@@ -297,6 +297,34 @@ describe('Process a file', () => {
         done();
       });
     });
+
+    describe('fs', () => {
+      it('reads and writes using a custom fs when provided', done => {
+        const before = 'abc';
+        let written;
+
+        const fs = {
+          readFile: (_fileName, _encoding, callback) => {
+            callback(null, before);
+          },
+          writeFile: (_fileName, data, _encoding, callback) => {
+            written = data;
+            callback(null);
+          },
+        };
+
+        transform({
+          files: 'test1',
+          fs,
+          processor: (input) => {
+            return input.replace(/b/, 'z');
+          },
+        }).then(() => {
+          expect(written).to.equal('azc');
+          done();
+        });
+      });
+    });
   });
 
   /**
@@ -725,6 +753,35 @@ describe('Process a file', () => {
       expect(results[2].file).to.equal('test3');
       expect(results[2].hasChanged).to.equal(false);
     });
+
+    describe('fs', () => {
+      it('reads and writes using a custom fs when provided', done => {
+        const before = 'a';
+        let written;
+
+        const fs = {
+          readFileSync: () => {
+            return before;
+          },
+          writeFileSync: (_fileName, data) => {
+            written = data;
+            return data;
+          },
+        };
+
+        const results = transform.sync({
+          files: 'test1',
+          fs,
+          processor: (input) => {
+            return input.replace(/a/, 'z');
+          },
+        });
+
+        expect(results[0].file).to.equal('test1');
+        expect(written).to.equal('z');
+        done();
+      });
+    });
   });
 
   describe('module export', () => {
diff --git a/lib/replace-in-file.spec.js b/lib/replace-in-file.spec.js
index eaccd71..754d731 100644
--- a/lib/replace-in-file.spec.js
+++ b/lib/replace-in-file.spec.js
@@ -416,6 +416,33 @@ describe('Replace in file', () => {
         done();
       });
     });
+
+    describe('fs', () => {
+      it('reads and writes using a custom fs when provided', done => {
+        const before = 'a';
+        let written;
+
+        const fs = {
+          readFile: (_fileName, _encoding, callback) => {
+            callback(null, before);
+          },
+          writeFile: (_fileName, data, _encoding, callback) => {
+            written = data;
+            callback(null);
+          },
+        };
+
+        replace({
+          files: 'test1',
+          from: /a/,
+          fs,
+          to: 'z',
+        }).then(() => {
+          expect(written).to.equal('z');
+          done();
+        });
+      });
+    });
   });
 
   /**
@@ -1149,6 +1176,33 @@ describe('Replace in file', () => {
       expect(results[0].numMatches).to.equal(0);
       expect(results[0].numReplacements).to.equal(0);
     });
+
+    describe('fs', () => {
+      it('reads and writes using a custom fs when provided', () => {
+        const before = 'a';
+        let written;
+
+        const fs = {
+          readFileSync: () => {
+            return before;
+          },
+          writeFileSync: (_fileName, data) => {
+            written = data;
+            return data;
+          },
+        };
+
+        const results = replace.sync({
+          files: 'test1',
+          from: /a/,
+          fs,
+          to: 'z',
+        });
+
+        expect(results[0].file).to.equal('test1');
+        expect(written).to.equal('z');
+      });
+    });
   });
 
   describe('module export', () => {