Drawing on categorization theory, this empirical study focuses on the critical role of congruency in prototypical brand extensions. A survey, measuring reactions to 18 proposed extensions involving six well-known brands (Xerox, Kleenex, Band-Aid, Scotch Tape, Coke, Sony Walkman), was administered to a sample of graduate students. For each prototypical brand, there were three proposed extensions: congruent, moderately congruent, and incongruent. Preliminary analyses, using a series of MANOVAs and post hoc pairwise difference tests, revealed robust variations across the six brands with respect to the level of extension congruency. Using multiple-item indicators for three latent constructs (attitude towards parent brand, extension fit, and extension success) along with two additional variables (prototypicality and manufacturing complexity), we developed and tested three structural equation models linking these aforementioned variables, for congruent, moderately congruent, and incongruent extensions. Results indicate that—irrespective of the level of extension congruency—perceptions of fit had the strongest influence on extension success. However, while parent brand affect directly and indirectly influenced success for congruent brand extensions, these parent-brand associations played no significant role for moderately congruent and incongruent brand extensions. Implications for brand managers and directions for future research are also discussed.