Over the past decade, the focus of much effort in systems development has evolved from the development of individual self- contained systems to the integration of large-scale systems of systems (SoS) that are constantly evolving to address new user needs. Because these types of systems of systems no longer have a single controlling authority, have components that are developed and evolve independently, and as a result cannot be specified by a top-down set of requirements, the methods for engineering them need to be modified from the methods for engineering traditional systems. This paper identifies the characteristics of SoS, proposes a SoS life cycle, and identifies some considerations for requirements engineering in an SoS environment.