Achieving acceptable quality of service in highly dynamic computing environments requires not only adaptation and reconfiguration of individual components of the system, but also collaboration among these components. To address these issues in adaptive multimedia applications, we propose COCA, a contract-based collaborative adaptation infrastructure. COCA uses contract specifications to describe the relationships among different system components, to generate code that realizes adaptive functionality, and to govern system-wide adaptive behavior during execution. COCA also provides a set of reusable adaptation services that enable legacy components to be integrated into an adaptive system. We demonstrate the use of COCA to construct an adaptive multimedia conferencing system from a collection of existing components that were not originally designed for interoperation.