This chapter reviews the research that defines fit (congruence) between the nonprofit and the firm participating in corporate social responsibility (CSR) initiatives and explores the conditions under which heightened fit may improve effectiveness. We explain the origins of CSR initiatives, and their growth and current popularity. We review the various typologies or categories offered by researchers, and the theoretical explanations of fit, including the commonality of fit (i.e., the overlap between brand and nonprofit), as well as complementarity of fit (i.e., the extent to which the nonprofit adds to the understanding or evaluation of the brand). The ways in which perceptions of fit may be heightened is reviewed, as is a relatively new exploration of the limits of greater fit, including the potential benefits of a lack of fit. Finally, the various consequences of fit are reviewed, as are future considerations.