diff --git a/README.md b/README.md
index 12e43a3..0576404 100644
--- a/README.md
+++ b/README.md
@@ -21,6 +21,56 @@
 $ docker-compose up -d api
 ```
 
+### Usage
+
+#### API Key
+
+A ``master`` API key is created at the first launch of the application, API keys are stored in ``./api-keys.json`` file.
+
+An API key gives access to more fields returned by the API, for examples:
+
+```json
+{
+  "a7e666ed93d877d9a4f9": {
+    "name": "my-api-key",
+    "fields": {
+      "employees": ["*"],
+      "structures": ["field", "field.subfield", "field.*"],
+    }
+  },
+  "939500eae15165ab4b8a": {
+    "name": "my-api-key",
+    "fields": {
+      "employees": ["field"],
+    }
+  },
+  "e24d1167135d65bfcaad": {
+    "name": "my-api-key",
+    "fields": {
+      "structures": ["field.*"],
+    }
+  }
+}
+```
+
+``employees`` *(type: Array)* and ``structures`` *(type: Array)* are optional, the fields returned by the API will be the default ones (cf: [Environment variables - **FIELDS_NAME**](#fieldsName)).
+
+The values of its tables can be:
++ ``*`` gives access to all the fields.
++ ``field`` gives access to a field
++ ``field.subfield`` gives access to a subfield
++ ``field.*`` gives access to all subfields
+
+There are two ways to use an api key, by the **header** ``x-api-key`` (in lowercase) or the **url parameter** ``apiKey``.
+
+Examples: 
+
+```bash
+$ curl -H "x-api-key: 6596ae25fdfdc83b85a7" http://localhost:3000/employees
+
+$ curl http://localhost:3000/employees?apiKey=6596ae25fdfdc83b85a7
+```
+
 ## Development
 
 ```bash
@@ -40,15 +90,15 @@
 
 | Name | Type | Description |
 | --- | --- | --- |
-| APP_PORT | Integer | Application port ``(default: 3000)`` |
-| ELASTICSEARCH_SCHEME | String | URI scheme ``(default: https)`` |
-| ELASTICSEARCH_HOST | String | ElasticSerach host ``(default: localhost)`` |
-| ELASTICSEARCH_PORT | Integer | ElasticSerach port ``(default: 9200)`` |
-| ELASTICSEARCH_USERNAME | String | ElasticSerach host ``(default: elastic)`` |
-| ELASTICSEARCH_PASSWORD | String | ElasticSerach host ``(default: changeme)`` |
-| RESEDA_EMPLOYEES_INDEX | String | Reseda employees index name ``(default: employees)`` |
-| RESEDA_STRUCTURES_INDEX | String | Reseda structures index name ``(default: structures)`` |
-| PARAMS_MAX_SIZE | Integer | Query max size ``(default: 1000)`` |
-| PARAMS_DEFAULT_SIZE | Integer | Query default size ``
-(default: 25)`` |
-| RESEDA_DOC_URL | String | OpenAPI URL ``(default: docs)`` |
+| APP_PORT | <span style="color: #27ae60;">Integer</span>  | Application port ``(default: 3000)`` |
+| ELASTICSEARCH_SCHEME | <span style="color: #2980b9;">String</span> | URI scheme ``(default: https)`` |
+| ELASTICSEARCH_HOST | <span style="color: #2980b9;">String</span> | ElasticSerach host ``(default: localhost)`` |
+| ELASTICSEARCH_PORT | <span style="color: #27ae60;">Integer</span>  | ElasticSerach port ``(default: 9200)`` |
+| ELASTICSEARCH_USERNAME | <span style="color: #2980b9;">String</span> | ElasticSerach host ``(default: elastic)`` |
+| ELASTICSEARCH_PASSWORD | <span style="color: #2980b9;">String</span> | ElasticSerach host ``(default: changeme)`` |
+| RESEDA_EMPLOYEES_INDEX | <span style="color: #2980b9;">String</span> | Reseda employees index name ``(default: employees)`` |
+| RESEDA_STRUCTURES_INDEX | <span style="color: #2980b9;">String</span> | Reseda structures index name ``(default: structures)`` |
+| PARAMS_MAX_SIZE | <span style="color: #27ae60;">Integer</span>  | Query max size ``(default: 1000)`` |
+| PARAMS_DEFAULT_SIZE | <span style="color: #27ae60;">Integer</span> | Query default size ``(default: 25)`` |
+| RESEDA_DOC_URL | <span style="color: #2980b9;">String</span> | OpenAPI URL ``(default: docs)`` |
+| <span id="fieldsName">FIELDS_NAME</span> | <span style="color: #f39c12;">JSON</span> | Default returned fields ``(default: { "employees": [ "nomUsage", "prenom", "UUIDPersonne", "affectationAdministrative.codeUnite" ], "structures": [ "codeUnite", "intituleUnite" ] })`` |