Graphical user interfaces are difficult to implement because of the essential concurrency among multiple interaction devices, such as mice, buttons, and keyboards. Squeak is a user interface implementation language that exploits this concurrency rather than hiding it, helping the programmer to express interactions using multiple devices. We present the motivation, design and semantics of squeak . The language is based on concurrent programming constructs but can be compiled into a conventional sequential language; our implementation generates C code. We discuss how squeak programs can be integrated into a graphics system written in a conventional language to implement large but regular user interfaces, and close with a description of the formal semantics.