Systems Integration is an activity omnipresent in almost all of systems engineering and management. Often the term lacks precise definition and is used in different ways and for different purposes in the engineering of systems. This paper presents an overview of systems integration at all levels of system engineering and management. The goal of the paper is to present the various views of systems integration including lifecycle, architecture, process, interface, and enterprise, as well as product integration involving software and hardware. We examine systems engineering process, including several process models in an attempt to reveal the activities associated with integration during the engineering of systems. We review the characteristics of interfaces between systems of systems and users that generally give rise to many systems integration concerns. Without very careful effort at developing an appropriate architecture for a system, there will be little hope of integration. We discuss the role of systems management and knowledge integration in architecting for systems integration. Also presented are several contemporary perspectives on system architectures and their role in the engineering and integration of systems.