Increasing competition is leading major grocery retailers to improve the effectiveness of their offer’s differentiation adopting an assortment planning’s approach based on the concept of product category. In this framework, the paper aims to investigate merchandise category’s definition criteria and the real level of adoption and implementation of the “category management philosophy” in some of the most important Italian grocery retailing firms, with particular attention to the implications concerning the introduction of new products. This was revealed through an empirical research based on in-depth interviews with senior managers, aimed to identify the retailer’s rationality behind product categories’ definition and composition.