The authors investigated the empirical generalizability of existing brand extension research results. Using a comprehensive data set compiled from two large-scale consumer samples and panel data, they address the generalizability of empirical findings (1) beyond the lab to conditions with real extensions, (2) across fast-moving consumer goods product categories, (3) across different types of parent brands, (4) across respondents (students versus non-students), and (5) across success measures (attitude versus market-based success measures). Many important results of brand extension research generalize, to a certain extent, across all five areas of empirical generalization. However, some notable differences occurred (e.g., between different types of parent brands). Beyond its implications for brand extension research, this study underscores the need for the development of empirical generalizations that provide sound knowledge to practitioners and academics.