GUIKit provides a higher-level Mathematica expression syntax for defining a graphical user interface along with a runtime environment for managing and deploying these reusable definitions. GUIKit simplifies the construction and layout of common user interface programming and eliminates the need to write code using the underlying Java programming language. GUIKit allows Mathematica users to quickly define interfaces as Mathematica expressions and to program the logic of these user interfaces with familiar Mathematica programming language constructs.