We present a software architecture model of adaptation in CBR. A software architecture is defined by its components and their connectors. We present a software architecture for CBR systems based on three components (a task description, a domain model, and adaptors) connected by a type of connectors called bridges. Adaptors are basic inference components that perform specific transformations to cases. Two kinds of adaptors are introduced: domain adaptors (d-adaptors) and case- based adaptors (c-adaptors). Adaptors are applied to a given problem, performing search until a sequence of adaptor instantiations is found such that a solution is achieved. Thus, in the ABC architecture adaptation is viewed as a search process on the space of adaptors. The ABC components have been used in the SaxEx application, a CBR system for generating expressive musical phrases.