Products vary in the degree to which they are members of product categories. Understanding how product attributes influence product categorization is important for marketing decisions about product development, brand extension, and product positioning. Existing attribute-level measures of product attitude emphasize evaluation rather than categorization. Measures of category membership either are at the overall product level or emphasize feature-based attributes, whereas product attributes are often continuous in nature. A comprehensive approach for conceptualizing product categorization is provided by fuzzy set theory, with its basic postulate that no clear boundaries exist between members and non-members of a fuzzy set. The authors conceptualize product categories as fuzzy sets in which products have degrees of membership along specific attributes, argued to arise as a result of the degree to which a product possesses an attribute. Two fuzzy set-based measures of category membership are developed that assess gradedness of category membership at the attribute level, which is then combined across attributes to reach overall measures of gradedness for a product. The results of a study demonstrate the validity of these measures and illustrate important attribute-level analyses that are not possible with existing approaches.