diff --git a/data_format.md b/data_format.md
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--- a/data_format.md
+++ b/data_format.md
@@ -1,2 +1,78 @@
 # Data Format
-TODO  
+
+Recording and annotation data are stored in a JSON format. Each recording info
+with its corresponding annotations are stored in a separate file, with the file 
+name corresponding to the recording ID and suffixed with `.json`.
+
+## Recording Info
+
+The recording info is stored in the top-level of the JSON file. The following
+fields are included:
+
+- `id`: The recording ID, which is the file name (with the `.wav` suffix).
+- `duration`: The duration of the recording in seconds.
+- `class_name`: A field to store a file level class name.
+- `time_exp`: The time expansion factor used to generate the recording. If not
+  specified, the default value is 1.
+- `annotated`: A boolean field (true/false) to indicate whether the recording
+  has been fully annotated.
+- `issues`: A boolean field (true/false) to flag any issues with the recording.
+  This is used to indicate that the recording or annotations should be reviewed.
+- `notes`: A field to store any notes about the recording.
+- `annotation`: A list of annotations for the recording.
+
+## Annotation
+
+Annotations are made in the form of bounding boxes. Each annotation is stored as
+a dictionary with the following fields:
+
+- `class`: The class name associated to the annotation.
+- `start_time`: The start time of the annotation in seconds.
+- `end_time`: The end time of the annotation in seconds.
+- `individual`: The individual ID associated to the annotation.
+- `event`: The event type associated to the annotation.
+- `low_freq`: The low frequency of the annotation in Hz.
+- `high_freq`: The high frequency of the annotation in Hz.
+
+## Example
+
+```json
+{
+  "id": "20170701_213954-MYOMYS-LR_0_0.5.wav",
+  "duration": 0.5,
+  "class_name": "Myotis mystacinus",
+  "annotated": true,
+  "issues": false,
+  "notes": "",
+  "time_exp": 1,
+  "annotation": [
+    {
+      "class": "Myotis mystacinus",
+      "end_time": 0.151696,
+      "event": "Echolocation",
+      "high_freq": 105957.03125,
+      "individual": "0",
+      "low_freq": 28564.453125,
+      "start_time": 0.146366
+    },
+    {
+      "class": "Myotis mystacinus",
+      "end_time": 0.028696,
+      "event": "Echolocation",
+      "high_freq": 95214.84375,
+      "individual": "0",
+      "low_freq": 34667.96875,
+      "start_time": 0.025006
+    },
+    {
+      "class": "Myotis mystacinus",
+      "end_time": 0.059036,
+      "event": "Echolocation",
+      "high_freq": 107910.15625,
+      "individual": "0",
+      "low_freq": 30517.578125,
+      "start_time": 0.053296
+    }
+  ]
+}
+```