This research explores links between constraint development in creative processes and end user development in environments for creative tasks. A process model describing the development of constraints in creative tasks is presented. To ground the research, support for the user development of musical instruments in a collaborative composition environment is developed, and the use of the system analysed. We find evidence that the development of tangible constraints in the interface has value to users, particularly in focusing collaborative ideation. Keywords Creativity, Constraint, Collaboration, Music Creation