Environmental changes aimed at encouraging walking or cycling may promote activity and improve health, but evidence suggests small or inconsistent effects in practice. Understanding how an intervention works might help explain the effects observed and provide guidance about generalisability. We therefore aimed to review the literature on the effects of this type of intervention and to understand how and why these may or may not be effective. We searched eight electronic databases for existing systematic reviews and mined these for evaluative studies of physical environmental changes and assessed changes in walking, cycling or physical activity. We then searched for related sources including quantitative or qualitative studies, policy documents or reports. We extracted information on the evidence for effects ('estimation'), contexts and mechanisms ('explanation') and assessed credibility, and synthesised material narratively. We identified 13 evaluations of interventions specifically targeting walking and cycling and used 46 related sources. 70% (n = 9 evaluations) scored 3 or less on the credibility criteria for effectiveness. 6 reported significant positive effects, but higher quality evaluations were more likely to report positive effects. Only two studies provided rich evidence of mechanisms. We identified three common resources that interventions provide to promote walking and cycling: (i) improving accessibility and connectivity; (ii) improving traffic and personal safety; and (iii) improving the experience of walking and cycling. The most effective interventions appeared to target accessibility and safety in both supportive and unsupportive contexts. Although the evidence base was relatively limited, we were able to understand the role of context in the success of interventions. Researchers and policy makers should consider the context and mechanisms which might operate before evaluating and implementing interventions.