Mounting research evidence suggests that the shift to contingent work arrangements in industrialized countries is having serious adverse effects on the health of workers, both directly and indirectly (by undermining regulatory and other protections). The authors place this research, and the issues surrounding it, in a comparative historical context. Extensive use of precarious employment is not essentially new. It was a characteristic feature of most if not all industrialized societies in the 19th and early 20th centuries. Though the two phases are not identical, historical comparisons are instructive for understanding recent experiences and ways of addressing them. The authors also make comparisons with the developing world, where the informal sector typically accounts for over half the workforce. Such comparisons are instructive in indicating the consequences of a shift to more precarious patterns of employment and disorganized work settings. There is also good evidence that precarious employment is expanding in the developing world. The growing precarious employment in both industrialized and developing countries is interconnected, and the authors identify a number of the mechanisms affecting workers' health.