This chapter adopts an institutional lens in considering both the nature of health systems and ways of strengthening them. First, it reviews five widely known health system conceptual frameworks, highlighting the different types of agents, organizations, and organizational arrangements that are embedded within each framework, and identifying the nature of relationships among actors, and the institutions each identifies or implies as underpinning these relationships. Second, it presents recent thinking on health system governance — a central, but less considered, function of every health system. Third, three complementary bodies of theory (organizational and policy implementation theory, and systems thinking) that draw on institutional perspectives in considering organizational functioning and change, are briefly discussed and applied in critique of the health system frameworks. The critique highlights the dominance of a mechanical perspective of organizational functioning within existing frameworks, and a primarily command and control approach to health system strengthening. Finally, two alternative approaches to supporting change within health systems, both of which acknowledge complexity and seek institutional change, are introduced: soft systems methodology and strengthening trust-based relationships.