diff --git a/SPEC.md b/SPEC.md
index 76d6b56..c9f5fab 100644
--- a/SPEC.md
+++ b/SPEC.md
@@ -14,11 +14,31 @@ The `type` field is either `file` or `dir`.
 A file object has the following fields.
 
   - `type`: String with value of `'file'`.
-  - `data`: TODO: Define structure for file content data.
+  - `data`: Array of `file-data`.
   - `size`: Integer. Cumulative size of `data`.
 
 The `type` field must be set to `file`.
 
+### `file-data`
+
+File data is an Array. Each element is an Array with only 2 elements (Tuple).
+
+  - 0: Array. Tuple containing two integers, the `start` and `length` offsets of the content.
+    - 0: Integer: start offset.
+    - 1: Integer: length of part(s).
+  - 1: Link. Either a `raw` link or a link to another node formatted as `file-data`.
+
+```javascript
+[
+  [ [0, 1024], Link ],
+  [ [1025, 1002], Link ]
+]
+```
+
+Implementations are encouraged to use nested `file-data` array nodes through links for files
+with many chunks in order to limit the size of the serialized node. No strict boundary is
+set but it is typical to try and limit node size to less than one megabyte.
+
 ## IPLD `dir`
 
 A directory object represents a directory.