This article focuses on a crucial aspect of the Argentine labor movement that, in spite of its importance, has not been adequately examined by the existing historiography: the high degree of union structure penetration at the shop-floor level through means of shop-stewards and comisiones internas (CI). First, it provides a basic definition of shop-stewards and CI for the Argentine case. Second, it briefly analyzes the history of these bodies from the early 1940s to the early 1980s, taking into account the most important structural transformations during this period, in particular the transition from an economic model driven by import-substituting industrialization from the 1930s to the mid-1970s to a process of deindustrialization from that moment onward. Third, it contends that these shop-stewards and CI experienced tensions and contradictions, some of which were related to those present within the Argentine industrial working class. Finally, it underlines the importance of taking into account this rich history of shop-floor organization to understand the strength of the Argentine labor movement in this period.