As systems continue to grow in scale and complexity, systems integration (SI) has become a key concern. This is especially the case in defense and aerospace. SI involves interfacing and enabling the interactions of component elements to collectively provide the functionality needed by the system to accomplish its goals. SI is part of the overall system development life cycle. SI increases in complexity when there are legacy systems that need to be integrated, and when humans are an integral part of the system. An added layer of complexity is introduced when the system has to exhibit resilience and adaptability in the face of contingencies in the operational environment. This paper addresses key perspectives and challenges in SI. Specifically, it presents the integration continuum, ranging from loose to tight integration. It presents a SI ontology that captures the key issues and concerns in a standard language. It also presents various categories of integration and their unique challenges. This paper is intended to clarify various types of integration and to stimulate new ways of thinking about SI.