Women have organized around their gendered identity to accomplish political goals both inside and outside legislatures. Formal and informal institutional norms shape the form this collective action takes and whether it is successful. What, then, are the favorable conditions for organizing women's caucuses inside legislatures? Using an original dataset and employing an event history analysis, we identify the institutional conditions under which women's caucuses emerged in the 50 US states from 1972 to 2009. Within a feminist institutional framework, we argue that women's ability to alter existing organizational structures and potentially affect gender norms within legislatures is contextual. Although we find that women's presence in conjunction with Democratic Party control partially explains women's ability to act collectively and in a bipartisan way within legislatures, our analysis suggests that institutional-level variables are not enough to untangle this complicated phenomenon. Our work explains how gender and party interact to shape legislative behavior and clarifies the intractability of institutional norms while compelling further qualitative evidence to uncover the best conditions for women's collective action within legislatures.