The Collaboration for Environmental Evidence (w'w'w.environmentalevidence.org) has published systematic reviews of the effect of protected areas, payment for ecosystem services and aspects of forest management on various human welfare, habitat and species preservation outcomes (Baylis et al. Including a broader variety of approaches to evaluate effectiveness, the OECD DAC Evaluation Resource Centre (DEReC) has collected over 3000 evaluation reports from its member’s development agencies, including on environment, forestry and agriculture (www .oecd.org/derec'). Theory-based evaluation aims to ascertain, through a mixed methods approach, the question of why a programme or policy did or did not work. It examines the assumptions underlying the causal chain from inputs to outputs, outcomes and impacts.