We studied CAD system users to find out how they use the sophisticated customization and extension facilities offered by many CAD products. We found that users of varying levels of expertise collaborate to customize their CAD environments and to create programmatic extensions to their applications. Within a group of users, there is at least one local expert who provides support for other users. We call this person a local developer. The local developer is a fellow domain expert, not a professional programmer, outside technical consultant or MIS staff member. We found that in some CAD environments the support role has been formalized so that local developers are given official recognition, and time and resources to pursue local developer activities. In general, this formalization of the local developer role appears successful. We discuss the implications of our findings for work practices and for software design.