**Abstract**

This chapter discusses existing and future trends on the design and build of "Modular" and "Open" satellite Bus and mission payload along with practical design issues associated with the use of Modular Open System Approach (MOSA). Existing modular Bus and mission payload architectures for typical commercial, civilian, and military satellite systems will be discussed. The chapter provides space industry views on "Open" versus "Close" interfaces design and addresses the challenges associated with open interfaces using Open System Architecture (OSA) approach using MOSA principles. The system interfaces discuss in this chapter include (i) internal to satellite Bus and mission Payload (PL), (2) between satellite Bus and mission payload, and (3) external to both satellite Bus and mission payload.

**Keywords:** open system architecture, Modular Open System Approach (MOSA), satellite Bus, mission payload (PL), modular architecture, open Interface, close Interface, modular satellite Bus, modular Mission payload
