The processes of clinical decision making form a major part of the activities associated with the delivery of health care. Dealing with complexity, such as is found in health care delivery, is the essence of systems science—taking a systems view of complex organizations and activities, their regulatory and control mechanisms, and the manner in which decisions can be made in order to achieve enhanced performance (Flood and Carson, 1993). In this chapter, the nature of systems and the manner in which they may be regulated and controlled are outlined. This provides the basis for the control-system model of therapeutics that is presented in Chapter 4.