The move to component-based development (CBD) is driven by a number of important requirements with respect to the agile assembly of application systems from pre-developed parts. However, to make CBD a reality requires a deeper understanding of the fundamental concepts of CBD and how those concepts can be embodied in tools and techniques. The most basic questions are in respect to what constitutes a component, how appropriate components can be found, and how to assemble components when building an application system. This paper addresses these questions and makes the case for more rigorous descriptions of component behaviors through the use of domain and specification models which allow interfaces and implementations of components to be more easily understood, analyzed and combined.