‘Complexity theory’ is one of the most over-used and misapplied terms in contemporary healthcare literature. Yet complexity is not, in and of itself, a theory. It would be better to talk about complexity thinking—that is, having a general understanding of and attitude towards the world which acknowledges (for example) multiple components which interact and are interdependent, adaptation and emergence, and unpredictability. Several academic disciplines have, largely independently of one another, discovered that the world is complex in such ways and developed theories, approaches and methods for exploring that complexity. In this chapter, the author describes the historical origins of four such disciplines—information systems (cybernetics), living systems (biology, ecology and human sciences), organisational systems (organisational sociology) and political systems (political philosophy). Examining the commonalities and differences among these traditions, she distils some implications for managing complexity in primary care research: (1) Classify the challenge (simple, complicated or complex?); (2) understand the system (don’t simplify—complexify); (3) represent the system (eg draw maps, tell stories); (4) take action (eg muddle through, build partnerships, empower frontline actors); and (5) learn and develop (try things out, deliberate, change direction as needed).