Market systems development (MSD) programmes aim to influence private actor behaviour to promote markets that work better for the poor. This article aims to inform theory-based evaluation (TBE) of such programmes, arguing that a stronger analysis of market actor behaviour change is needed. It proposes a ‘behaviour change framework’ (BCF), building on recent advances in the TBE literature. These focus attention on behaviour change as contingent on the alignment of actor capability, motivation, and opportunity, influenced by the meso and macro contexts. The article applies the BCF to three theory-based MSD evaluations to illustrate its applicability and draw lessons on its use. The BCF can be used to identify evidence gaps and support more compelling explanations of what worked and under what conditions. Such evidence can inform future MSD programmes, and enable them to better stimulate systemic change in line with poverty reduction.