The objective of this chapter is to provide guidelines for designing the functional view of a software system. It covers the refinement of the context-level use case model, the identification of functional components, the allocation of these components to functionality placeholders within a schematic architecture, the design of component interfaces, and the design of functional variability through the identification of variation points and the definition of valid variants. Together, these guidelines enable architects to construct a functional architecture that is coherent, modular, and aligned with both system requirements and architectural principles.