This article proposes ways to use programme theory for evaluating aspects of programmes that are complicated or complex. It argues that there are useful distinctions to be drawn between aspects that are complicated and those that are complex, and provides examples of programme theory evaluations that have usefully represented and address both of these. While complexity has been defi ned in varied ways in previous discussions of evaluation theory and practice, this article draws on Glouberman and Zimmerman’s conceptualization of the differences between what is complicated (multiple components) and what is complex (emergent). Complicated programme theory may be used to represent interventions with multiple components, multiple agencies, multiple simultaneous causal strands and/or multiple alternative causal strands. Complex programme theory may be used to represent recursive causality (with reinforcing loops), disproportionate relationships (where at critical levels, a small change can make a big difference – a ‘tipping point’) and emergent outcomes.