Creative tasks are performed within constraining structures. However, the exploration and development of these structures is central to the creative process. This paper summarizes research on the role of constraint in creative tasks, defines the role of constraint development in a model of the creative process, and classifies types of constraint in order to inform the design of environments for creative tasks. Links between constraint development and end user development in software environments are explored through analysis of the design and use of Music Builder, a prototype for the user development of musical instruments in a collaborative composition environment. Conclusions include the value of scaffolding as a metaphor in design, the utility of collaborative constraint development to negotiation, and coordination and the value of sharing structures in collaborative development.