This is a guide to the legal framework for emissions trading under the cap-and-trade system created and adhered to under the Western Climate Initiative (WCI). This guide is intended to serve three aims. First, the guide is an overview of the WCI cap-and-trade system for emissions trading by current users of the system; potential industry participants; state, provincial and municipal governments; academic institutions; and members of civil society. Second, the guide’s aim is to foster learning among domestic and international actors interested in North America’s collective response to climate change and highlights one attempt to combat climate change through a subnational cap-and-trade system on the continent. Third, during the course of research for this guide in 2018, the province of Ontario linked its WCI-inspired cap-and-trade system with that of California and Quebec and six months later delinked its system, eventually terminating it altogether and announcing its intention to withdraw from the WCI. A third purpose of this guide is therefore to serve as an account of Ontario’s short-lived cap-and-trade system and its brief experience with linkage.