This systematic review by Waddington, Sonnenfeld and colleagues examined high-quality evidence from 35 citizen engagement programmes in low- and middle-income countries. These programmes promoted citizen engagement in service delivery through four routes: participation, inclusion of marginalised groups, transparency and/or citizen efforts to ensure public service accountability; and collectively, PITA mechanisms. Interventions that improved direct engagement between service users and providers are often effective in stimulating active citizen engagement in service delivery, and realising improvements in access to services and service provision quality. However, in the absence of complementary interventions to address bottlenecks around service provider supply chains and service use, citizen engagement interventions alone may not improve key well-being outcomes for target communities. In addition, interventions to improve governance by increasing citizen pressures on politicians to hold service providers to account do not usually influence service delivery.