PICASSO is a graphical user interface development system that includes an interface toollkit and an application framework. The application framework provides high-level abstractions including modal dialog boxes and non-modal frames and panels similar to conventional programming language procedures and co-routines. These abstractions can be used to define objects that have local variables and that can be called with parameters. PICASSO also has a constraint system that is used to bind program variables to widgets, to implement triggered behaviors, and to implement multiple views of data. The system is implemented in Common Lisp using the Common Lisp Object System and the CLX interface to the X Window System. Keywords: Graphical User Interface Development Environment, Application Framework, User Interface Toolkit, User Interfaces.