Complexity-resulting from interactions among many component parts-is a property of both the intervention and the context (or system) into which it is placed. Complexity increases the unpredictability of effects. Complexity invites new approaches to logic modeling, definitions of integrity and means of standardization, and evaluation. New metaphors and terminology are needed to capture the recognition that knowledge generation comes from the hands of practitioners/implementers as much as it comes from those usually playing the role of intervention researcher. Failure to acknowledge this may blind us to the very mechanisms we seek to understand. Researchers in clinical settings are documenting health improvement gains made as a consequence of complex systems thinking. Improvement science in clinical settings has much to offer researchers in population health.
Keywords: complexity; evaluation; implementation; improvement science; intervention research; intervention theory; metaphor.