Voluntary participation is a long-standing tenet of environmental conflict resolution, prescribed at a time when the eventual reach and character of ECR could only be imagined. Today, ECR processes are ubiquitous, varied, and not always voluntary. Voluntary engagement involves choice and perception of fairness instilling commitment, good faith, respectful exchange, and legitimacy. These essential process qualities can be compromised if participation is mandated, yet mandates are sometimes necessary. This disconnect between theory and practice has been extensively examined for court-connected ADR involving civil disputes, but not for ECR involving public disputes. Mediator experience with mandated ECR processes warrants attention. V oluntary participation is a long-standing tenet of the environmental conflict resolution (ECR) field, prescribed at a time when the eventual reach and character of ECR could only be imagined. Today, ECR processes are ubiquitous, varied, and not always voluntary. However, the voluntary tenet has never been revisited; nor have the implications for practice been explicitly examined for those processes in which parties do not engage voluntarily. This article attempts to unravel the logic implicit in the longstanding prescription that collaborative ECR processes be voluntary. It asks which qualities associated with a voluntary process have made it a central tenet of ECR theory and what the shortcomings are when a process is compelled or mandated instead. In other words, what might be lost if voluntary engagement is compromised? Finally, it assesses the implications for