Lay participation in health care decision making has attracted increasing interest in Canada, with numerous provincial government reports advocating this initiative. Interest stems from a number of factors. Among them is a growing recognition that patient preferences ought to be incorporated into decision making that involves individual treatment choices. Another factor is the desire to increase public accountability for decisions on the allocation of health care resources, in order to make providers more accountable to the communities they serve. There is, however, still considerable confusion over what lay participation really means. In addition, little consideration has been given to whether and how lay participation can lead to better decision making and the criteria by which it should be judged. This article presents a framework based on decision-making domains, role perspectives, and levels of participation and is intended as an initial step toward providing greater conceptual clarity regarding the key dimensions and goals of lay participation in health care decision making.