In this article, I explain why pro-labor reforms succeed or fail. Focusing on the cases of Argentina and Chile, I show that labor reforms are more successful in extending trade union rights when unions successfully build associational power and employers are less able to do so. Consistent with this argument, a quantitative analysis of time-series cross-sectional data from 78 countries suggests that the level of class power disparity is negatively correlated with the extension of workers' collective rights. At the end of the article, I discuss how these results have implications for the study of labor reforms and power resources.