On August 14, 1983, the United States and Mexico signed the landmark United States–Mexico Agreement on Cooperation for the Protection and Improvement of the Environment in the Border Area, better known as the La Paz Agreement, which remains the keystone agreement for bilateral cooperation on environmental protection in the border zone. This article reviews the agreement’s history and achievements, with particular emphasis on the post–North American Free Trade Agreement environmental cooperation programs and its influence on other joint agencies with environmental mandates in the border region. It then reviews more recent implementation developments and reflects on political conditions shaping La Paz implementation. It concludes noting that the La Paz Agreement affirms the importance of certain institutional design principles for successful international environmental institutions and arguing for strengthening binational commitment to the La Paz Agreement.