diff --git a/README.md b/README.md
index 78b622a..e0d25b8 100644
--- a/README.md
+++ b/README.md
@@ -30,7 +30,7 @@ print tally(); //3
 
 * Simple C-like syntax
 * Bytecode intermediate compilation
-* Optional, but robust type system
+* Optional, but robust type system (including `opaque` for arbitrary data)
 * functions and types are first-class citizens
 * `import` and `export` variables from the host program
 * Fancy slice notation for strings, arrays and dictionaries
diff --git a/types.md b/types.md
index 07d0472..ea4c84e 100644
--- a/types.md
+++ b/types.md
@@ -19,6 +19,7 @@ The types available are:
 | dictionary | n/a | A series of key-value pairs stored in a hash-table, indexable with the keys |
 | function | fn | A chunk of reusable code, which can potentially return a value of some kind |
 | type | type | The type of types |
+| opaque | opaque | Arbitrary data passed from the host, which Toy can't natively understand |
 | any | any | Can hold any value |
 
 ## Specifying Types For Variables
@@ -93,3 +94,17 @@ var t = decide(true);
 
 var number: t = 0; //what if it had been false?
 ```
+
+## Opaque Data
+
+Sometimes, you may need to pass data through Toy that Toy can't normally handle. This data is called "opaque" data, and is passed around by reference rather than value. Anything can be passed in as opaque data, as long as it's represented as a void pointer in C.
+
+```c
+Literal opaque = TO_OPAQUE_LITERAL(&data);
+
+//...
+
+void* dataPtr = AS_OPAQUE(opaque);
+```
+
+Managing and cleaning up opaque data is a task left entirely to the host program.