Health systems are made up of multiple interacting components including organisations, people and actions. They perform several functions: delivering healthcare services; maintaining and improving health; protecting households from the costs of illness; enabling economic functioning; and shaping societal norms and values. They are also sites of competition and contestation between actors with different interests and visions.

This chapter outlines the complexity of health systems and explains how they can only be studied and fully understood through a multi-disciplinary lens. It unpacks the different dimensions of health systems complexity, the societal functions performed by health systems, and the contestation between different ideas, values and interest groups. It examines various health systems frameworks and typologies and how they can be used to describe and understand the performance of health systems. It discusses the open and contextual nature of health systems and their relationship with external factors, including shifts in international health policy-making. Finally, the chapter ends with a brief introduction of systems thinking which is discussed further in Chapter 2.