ABSTRACT: The Alberta oil sands represent tremendous economic growth and prosperity for Alberta and Canada but their development does not come without cost. Environmental Non-Governmental Organizations (ENGOs), specifically the Pembina Institute and Greenpeace, have brought significant attention to the environmental impacts of development. Their history and involvement in the oil sands can be divided into two phases, the first characterized by a collaborative relationship between Pembina Institute and development proponents. The second is characterized by a strategic evolution of the Pembina Institute and the emergence of Greenpeace. Resource Mobilization Theory and Political Opportunity Theory are applied and analyzed to provide an account for the evolution and emergence of ENGOs in Alberta’s oil sands.