Given the increasing interest in alliance portfolios, alliance portfolio diversity (APD) has been the focus of many recent studies. Yet, the performance consequences of APD—or of diversity in general—are neither theoretically clear nor empirically consistent. With meta-analytic analyses, we assess extant research on the APD–performance link. Across studies, APD has a positive impact on performance, although the level of analysis and how performance is measured influence the relationship. Going beyond conventional quantitative synthesis, however, we also systematically uncover patterns in how theoretical orientation and the operationalization of diversity moderate the APD–performance relationship. Our study serves as an invitation for future APD studies to employ more sophisticated theoretical and operationalization approaches as they expand our knowledge of diversity in alliance portfolios.