diff --git a/Expression.md b/Expression.md
new file mode 100644
index 0000000..aaf0031
--- /dev/null
+++ b/Expression.md
@@ -0,0 +1,5 @@
+Mapnik Expressions enable data-driven behavior in Mapnik. They are constructed from strings, can hold any kind of type of data, and are evaluated at render time. They are used by symbolizers like the [[TextSymbolizer]] to dynamically construct text labels for display and they are also the basis for how Mapnik [Filters](Filter) work.
+
+Recently, in Mapnik 2.1, expressions became enabled in the `transform` parser used to apply affine transformations to image and svg symbols. So, now you can not only, for example, rotate symbols with a transform like `transform="rotate(45)"` but the rotation can be data driven like `transform="rotate([field])"`
+
+For more info on the original design see: http://mapnik.org/news/2009/12/08/future_mapnik2/