Joint Systems are anticipated to play an increasingly important role in Defence. This paper provides a way of thinking about Joint Systems intended to support activities such as Joint Force Capability planning and management. The central concept is that of Systems Hierarchy. This concept is used to develop the core concepts of Joint Systems Thinking. An extension of the conceptual basis of conventional systems engineering is provided that deals with the uncertainty of requirements and the rapidly changing technology inherent in Joint Systems. The concept of system is also placed in an inheritance hierarchy that includes the concepts of application and federation. The usefulness of the concept of a system-of-systems is discussed within various contexts and an analogous inheritance hierarchy is provided for systems-of-systems. Finally, a portfolio as a construct that supports planning and investment decisions is defined and core concepts of portfolio management are discussed.