IMOS (InP membrane on silicon) is a platform-based approach to create indium phosphide (InP) nanophotonic integrated circuits. It uses monolithic integration of all photonic functions in the InP layers to leverage the most efficient optoelectronic processes and to remove optical interfaces between separately grown wafers. Using an optoelectronic material for the photonic components provides a powerful route to nanoscale miniaturization and circuit-level performance enhancements. In this chapter, we review the progress in the passive and active InP building blocks and highlight routes to miniaturization and performance scaling. We address the interfaces between building blocks, which are critical to a powerful platform approach. The platform approach enables customization by designers to create new components and circuits. The process design kit methodology is applied to provide a route to circuit design abstraction.