diff --git a/README.md b/README.md
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--- a/README.md
+++ b/README.md
@@ -14,3 +14,4 @@ can then be loaded into the æternity system.
 The basic modules for interfacing the compiler:
 
 * [aeso_compiler: the Sophia compiler](./docs/aeso_compiler.md)
+* [aeso_aci: the ACI interface](./docs/aeso_aci.md)
diff --git a/docs/aeso_aci.md b/docs/aeso_aci.md
new file mode 100644
index 0000000..c40956c
--- /dev/null
+++ b/docs/aeso_aci.md
@@ -0,0 +1,135 @@
+# aeso_aci
+
+### Module
+
+### aeso_aci
+
+The ACI interface encoder and decoder.
+
+### Description
+
+This module provides an interface to generate and convert between
+Sophia contracts and a suitable JSON encoding of contract
+interface. As yet the interface is very basic.
+
+Encoding this contract:
+
+```
+contract Answers =
+  record state = { a : answers }
+  type answers() = map(string, int)
+
+  stateful function init() = { a = {} }
+  private function the_answer() = 42
+  function new_answer(q : string, a : int) : answers() = { [q] = a }
+```
+
+generates the following JSON structure representing the contract interface:
+
+
+``` json
+{
+  "contract": {
+    "name": "Answers",
+    "type_defs": [
+      {
+        "name": "state",
+        "vars": [],
+        "typedef": "{a : map(string,int)}"
+      },
+      {
+        "name": "answers",
+        "vars": [],
+        "typedef": "map(string,int)"
+      }
+    ],
+    "functions": [
+      {
+        "name": "init",
+        "arguments": [],
+        "type": "{a : map(string,int)}",
+        "stateful": true
+      },
+      {
+        "name": "new_answer",
+        "arguments": [
+          {
+            "name": "q",
+            "type": "string"
+          },
+          {
+            "name": "a",
+            "type": "int"
+          }
+        ],
+        "type": "map(string,int)",
+        "stateful": false
+      }
+    ]
+  }
+}
+```
+
+When that encoding is decoded the following include definition is generated:
+
+```
+contract Answers =
+  function new_answer : (string, int) => map(string,int)
+```
+
+### Types
+``` erlang
+contract_string() = string() | binary()
+json_string() = binary()
+```
+
+### Exports
+
+#### encode(ContractString) -> JSONstring
+
+Types
+
+``` erlang
+ConstractString = contract_string()
+JSONstring = json_string()
+```
+
+Generate the JSON encoding of the interface to a contract. The type definitions and non-private functions are included in the JSON string.
+
+#### decode(JSONstring) -> ConstractString.
+
+Types
+
+``` erlang
+ConstractString = contract_string()
+JSONstring = json_string()
+```
+
+Take a JSON encoding of a contract interface and generate and generate a contract definition which can be included in another contract.
+
+### Example run
+
+This is an example of using the ACI generator from an Erlang shell. The file called `aci_test.aes` contains the contract in the description from which we want to generate files `aci_test.json` which is the JSON encoding of the contract interface and `aci_test.include` which is the contract definition to be included inside another contract.
+
+``` erlang
+1> {ok,Contract} = file:read_file("aci_test.aes").
+{ok,<<"contract Answers =\n  record state = { a : answers }\n  type answers() = map(string, int)\n\n  stateful function"...>>}
+2> Encoding = aeso_aci:encode(Contract).
+<<"{\"contract\":{\"name\":\"Answers\",\"type_defs\":[{\"name\":\"state\",\"vars\":[],\"typedef\":\"{a : map(string,int)}\"},{\"name\":\"ans"...>>
+3> file:write_file("aci_test.aci", Encoding).
+ok
+4> Decoded = aeso_aci:decode(Encoding).
+<<"contract Answers =\n  function new_answer : (string, int) => map(string,int)\n">>
+5> file:write_file("aci_test.include", Decoded).
+ok
+6> jsx:prettify(Encoding).
+<<"{\n  \"contract\": {\n    \"name\": \"Answers\",\n    \"type_defs\": [\n      {\n        \"name\": \"state\",\n        \"vars\": [],\n   "...>>
+```
+
+The final call to `jsx:prettify(Encoding)` returns the encoding in a
+more easily readable form. This is what is shown in the description
+above.
+
+### Notes
+
+The ACI generator currently cannot properly handle types defined using `datatype`.