This dissertation explores the boundaries of industrial unionism within and outside of the American Federation of Labor (AFL) in the struggle over what direction the American labor movement would take in the Progressive Era. The experiences of Iowa button worker and labor activist Pearl McGill in two nationally significant strikes between 1911 and 1912 enable us to see more clearly the nuances and ambiguities of these boundaries as industrial workers sought to build more inclusive unions. McGill’s advocacy for both the AFL-affiliated and industrially organized button workers in Iowa and the campaign of textile workers in Lawrence, Massachusetts, assisted by the Industrial Workers of the World (IWW), to organize on an industrial basis, shine a light on the conundrum faced by AFL leaders. The AFL and its craft union affiliates held fast to an anachronistic approach to organizing in an environment of rapid and technologically transformative industrialization in which the labor of women and ethnic and racial minorities was critical. The AFL’s early federal labor unions, for which Iowa button workers provide a case study, exemplify the strength of the impulse for unionization among mass production workers and show how AFL leaders fostered an institutional response to the growing demand for industrial unions while ensuring that craft unionists continued to dominate the AFL. The Women’s Trade Union League (WTUL) walked a fine, and sometimes precarious, line between its loyalty to the AFL and the demand of working women—notably in the garment and textile industries—for new, inclusive forms of organization. The strikes of women button workers and Lawrence textile workers illustrate the predicament faced by WTUL leaders. Pearl McGill’s short but prominent career as a youthful leader of the Muscatine button workers, a spokesperson for the WTUL, an advocate for women strikers, and a prominent activist with the IWW in Lawrence illuminates these tensions and the appeal of industrial unionism for young working women. This study elevates the importance of Progressive Era federal labor unions as a bridge connecting the local assemblies of the Knights of Labor of the 1880s to the industrial unions that would emerge in the 1930s. It examines the institutional history of the AFL and its bitter struggle with the Knights and establishes the link between the local assemblies of the Knights and the first generation of AFL-affiliated federal labor unions that provided a precedent for later industrial unions. The arc of industrial unionism in the United States can thus be seen as a long, interconnected movement rooted in the principles of general unionism embodied by the Knights and animated by the vital impulse for industrial unionism carried forward by industrially-organized workers of which Iowa button workers provide an important example.