An investigation of the demographic, labour market and welfare effects of unauthorized (or illegal) Mexican immigration to California since the 1980s. In this book, economist Enrico Marcelli examines whether the restrictive illegal immigration policy sentiment of the 1990s that led to the 1994 passage of California's Anti-Illegal Immigration Proposition 187, and to the federal-level Illegal Immigration and Welfare Acts two years later, is backed by reliable empirical evidence. This book examines the perception that unauthorized Mexican immigrants impact on the earnings and employment opportunities of other Californian residents and their comparative use of welfare programmes are analyzed. Contrary to this public perception and the direction of immigration policy in the mid-1990s, negligible earnings and employment effects are found, and unauthorized Mexican immigrants are found to have used public assistance programmes at lower rates and at lower levels than is typically assumed.