Issues of environment and environmental health involve multiple interests regarding e.g. political, societal, economical, and public concerns represented by different kinds of organizations and individuals. Not surprisingly, stakeholder and public participation has become a major issue in environmental and environmental health policy and assessment. The need for participation has been discussed and reasoned by many, including environmental legislators around the world. In principle, participation is generally considered as desirable and the focus of most scholars and practitioners is on carrying out participation, and making participation more effective. In practice also doubts regarding the effectiveness and importance of participation exist among policy makers, assessors, and public, leading even to undermining participatory practices in policy making and assessment.There are many possible purposes for participation, and different possible models of interaction between assessment and policy. A solid conceptual understanding of the interrelations between participation, assessment, and policy making is necessary in order to design and implement effective participatory practices. In this paper we ask, do current common conceptions of assessment, policy making and participation provide a sufficient framework for achieving effective participation? This question is addresses by reviewing the range of approaches to participation in assessment and policy making upon issues of environment and environmental health and some related insights from recent research projects, INTARESE and BENERIS.Openness, considered e.g. in terms of a) scope of participation, b) access to information, c) scope of contribution, d) timing of openness, and e) impact of contribution, provides a new perspective to the relationships between participation, assessment and policy making. Participation, assessment, and policy making form an inherently intertwined complex with interrelated objectives and outcomes. Based on experiences from implementing openness, we suggest complete openness as the new default, deviation from which should be explicitly argued, in assessment and policy making upon issues of environment and environmental health. Openness does not undermine the existing participatory models and techniques, but provides conceptual means for their more effective application, and opens up avenues for developing new kinds of effective participatory practices that aim for societal development through collaborative creation of knowledge.