This chapter focuses on developing system functions from previously defined system capabilities and activities, and how these functions lead to requirements for the system specification. Using the example of a satellite ground system, we demonstrate how the Department of Defense Architecture Framework (DoDAF) system view (SV) aids in defining the requirements necessary for functions such as command and telemetry operations. We also highlight the similarities between the SV and SysML Activity views, showing how these can be useful in transitioning from DoDAF to SysML as part of the overall program/project progression from architecture to system design.