Many have advocated for collaborative governance and the participation of citizens and stakeholders on the basis that it can improve the environmental outcomes of public decision making, as compared to traditional, top-down decision making. Others, however, point to the potential negative effects of participation and collaboration on environmental outcomes. This article draws on several literatures to identify five clusters of causal mechanisms describing the relationship between participation and environmental outcomes. We distinguish (i) mechanisms that describe how participation impacts on the environmental standard of outputs, from (ii) mechanisms relating to the implementation of outputs. Three mechanism clusters focus on the role of representation of environmental concerns, participants' environmental knowledge, and dialogical interaction in decision making. Two further clusters elaborate on the role of acceptance, conflict resolution, and collaborative networks for the implementation of decisions. In addition to the mechanisms, linking independent with dependent variables, we identify the conditions under which participation may lead to better (or worse) environmental outcomes. This helps to resolve apparent contradictions in the literature. We conclude by outlining avenues for research that builds on this framework for analysis.