This chapter investigates the origins and development of work-injury laws globally; those historically known as “workmen’s compensation” laws. The introduction of a national work-injury law ushers in the welfare state. Most research suggests modernization, industrialization and policy ideation caused cross-national variation in the timing and development of these laws. I argue that the experience and agency of workers and their families are also an important theoretical cause in this process. The challenge is that historical comparative data on worker agency across the globe do not exist. Until these data are collected, I propose an interim test of worker agency using socialist/Communist outcomes and slave production systems as two indicators of more or less capacity of workers to shape political outcomes. The test is counterfactual. If worker’s themselves were not a factor in the development of work-injury law, then there should be no clear association of socialism/Communism or year of abolition of slavery with the time it takes a country to enact a full-coverage blue-collar work-injury law. Just as technological and political systems diffused across the globe, so did ideas and tactics of worker movements. Therefore, working-class experience and agency should become part of mainstream theories of welfare state development and diffusion contingent upon improved future data and tests.