Various forms of public participation and stakeholder engagement have proliferated in Western democracies since the early 1990s as a result of both democratic demands from below, and tendencies to decentralisation and devolution from above. This tendency has been particularly notable in the realm of environmental governance, and has been driven by a widely-held belief that more participatory processes are ultimately more effective in producing decisions and policy outputs, as well as intermediate outcomes, conducive to environmental sustainability. As participatory governance has become more widespread, research and literature addressing this phenomenon has also flourished. Despite a now voluminous literature comprising thousands of case studies of public participation in environmental governance, attempts to systematically aggregate findings and consolidate knowledge on precisely how and under what conditions different forms of participatory governance work, remain rare. In this paper we present what have emerged as core hypotheses from the literature that address the relationship between (a) participation and the environmental standard of outputs, and (b) participation and the implementability of outputs. We disaggregate these hypotheses as far as possible, with the aim of isolating causal relations between important variables in the policy process. In addition, we take stock of contextual factors and boundary conditions with a bearing on different causal relations. We suggest that identification of the most important hypotheses on the relationship between participation and effectiveness is an important first step in generating and consolidating robust evidence on the ‘instrumental’ value of participatory modes of environmental governance. Such evidence should be of relevance to a wide range of researchers in the field of participatory governance, and of practical use to those that must balance environmental and democratic imperatives in designing and conducting participatory governance processes.