A software defined radio, as defined by the Federal Communications Commission (FCC) Notice of Proposed Rule Making (NPRM), FCC 00-430, is fully software driven and performs all digital signal processing using programmable digital signal processors, general purpose microprocessors, or field programmable gate arrays. All functions, modes, and applications can be reconfigured by software. More importantly, new capability can be added without hardware changes enabling a new generation of communication waveform developers to create new applications without intimate alliances with radio manufacturers. However, to be successful, the union of the application with the platform must be clearly understood with well-defined application programming interfaces (APIs). The well-defined interfaces form the contract between the application developer and the platform developer. The well-defined interfaces allow an application developer to develop an application once, that executes on many different manufacturers' platforms and provides the platform developer consistent interfaces under which platform technologies can evolve separately from the application. An application is independent of the platform it runs on if it is not coupled to a specific platform implementation, i.e., does not require platform unique hardware or software components for execution. The application accesses platform services through well-defined public interfaces that support the entire lifecycle of the application, i.e., from installation of the application through removal of the application from any given platform. This paper discusses SDR software architecture features, the need and definition of a common set of APIs and their extensibility into classes addressing factors such as cost and capability.