Depletion of stratospheric ozone emerged as a political concern in the early 1970s in the United States in the debate over the development of a commercial fleet of supersonic transports. In the mid- 1970s it became a major political issue with regard to the use of CFCs in aerosol spray cans, and in 1978 the United States banned the nonessential use of CFCs as aerosol propellants. Efforts at negotiating an international agreement controlling CFC use began in the 1980s and culminated in the 1987 Montreal Protocol. This paper traces the evolution of policy responses to stratospheric ozone depletion. The evolution of stratospheric ozone depletion policy can best be understood as a two-stage process. The first stage involves the emergence of stratospheric ozone depletion as a domestic issue in the United States and several other countries in the 1970s, while the second stage focuses on its transformation to an international issue in the 1980s. In addition to the emergence of stratospheric ozone depletion as an international political issue, three other factors are important in understanding the sources of the Montreal Protocol: (/) the evolving scientific understanding of the problem, (2) increasing public concern over the problem based on the threat of skin cancer and the discovery of the Antarctic ozone hole, and (3) the availability of acceptable substitutes for CFCs.