This paper discusses the fundamentals of architecting a major human spaceflight program and some of the lessons that can be learned from NASA's Constellation Program. This paper describes the Constellation program, whose primary objective focuses on development of a new generation of vehicles and systems to enable human exploration beyond Earth orbit. Constellation is made up of seven projects that are highly interdependent and is referred to in the NASA management system as a “tightly coupled” program. This paper will discuss the driving architectural priorities and characteristics for human exploration missions beyond earth orbit and how its building blocks are developed through initial capability missions to the International Space Station. The systems engineering challenges of simultaneously defining and developing systems that are interdependent will be discussed.