[en] Critical environmental zones are those ecosystems that are so degraded that the health or well-being of human inhabitants is threatened. A conceptual framework is presented for considering criticality and a rationale for a Canadian research project on critical zones. A model of pathways to criticality is outlined and some examples of environmental degradation in Canada are presented, including acid rain and greenhouse gas emissions. Societal response to, and public perception of, critical environmental zones is described. Media, format, and target audiences for output from a Canadian project are considered and some central scientific and policy questions are identified under such categories as environmental stresses, buffering capacity, indicators, human driving forces, and societal responses. An inventory of pertinent international and national activities is included. 53 refs., 8 figs., 3 tabs