Providing adequate health care to a nation's citizens is a challenge in every country. Despite large differences in wealth, health care organization, and health politics, both Mexico and the United States undertook similar efforts to expand primary care to previously underserved populations during the past 30 years. This study analyzes common antecedents, contexts of change, elements of the innovations, problems with entrenched interests, and resources that have allowed both programs to survive in difficult environments, We show that new forms of primary health care can face similar problems and prospects in very different countries because of similar political, bureaucratic, and economic limitations.