The extension of collective agreements can be a means to stabilize the coverage of collective bargaining. Much is known about the extension practice in individual countries. Knowledge about the motives for extension at the industry level, in turn, is limited. This article examines the distribution of extensions across industries in Germany. Applying a most-similar design and focusing on the construction industry, the retail trade, and the security sector, it compares how employers’ associations and trade unions deal with the extension and what interests they pursue in doing so. The analysis is based on the official extension directories, association publications, and interviews with representatives of trade unions and employers’ associations. It is demonstrated that actors are interested in the extension as it enables them to stifle ruinous competition based on under-cutting wages. However, it depends on the strategic choices made by employers’ associations whether this interest is enforced. In addition, there are sector-specific motives such as the protection of social security funds in the construction industry or efforts in the security sector to increase the sectors’ attractiveness. In the retail trade, in contrast, dissatisfaction with existing collective bargaining institutions and intensified competition led to the dissolution of collective agreements, the extension practice, and thus the cross-class coalition.