Social entrepreneurs develop market-driven ventures to produce social change; some succeed while others fail. This research advances our understanding of start-up behaviors of ventures that span nonprofit and for-profit institutional boundaries. A rigorous qualitative study of 23 social ventures reveals that entrepreneurs employ a blend of nonprofit and business venture behaviors, suggesting the importance of contextual factors. Only selective behaviors from each institution differentiate the successful from the struggling ventures. But while the higher level organizing tasks and activities of successful and struggling ventures may appear similar, fine-grained analyses of their behaviors show stark differences, emphasizing the need for such analyses.