His research focuses on informal housing and slums in developing countries and Third World-like housing conditions (including colonias, unpermitted trailer parks, and illegal garage apartments) in the United States. He is particularly interested in understanding the nature and necessity of informal housing, and strategies for upgrading and improving living conditions in unregulated housing. His work also examines how planners and urban designers in both developing and developed countries can learn from the everyday and informal city. Initially, he was known for his research in developing countries but now his work in California is also recognized.