This research explored the development of children's use of multiple conceptual organizations (thematic, taxonomic) in sorting sets of pictures. Experiment 1 revealed that between 5 and 9 years, two forms of categorical flexibility can be distinguished: Response and conceptual flexibility. It appeared that children's multiple sorts do not necessarily reflect the use of different conceptual organizations. Such lag was mainly due to a difficulty of access to taxonomic representations, specifically in the younger age groups. Therefore, Experiment 2 investigated the development of taxonomic representations using an original approach requiring participants to decide whether new items could be included into an existing taxonomic sort. This approach showed that taxonomic representations were only gradually differentiated from thematic-and perceptual ones over the 5 to 10 years period. The discussion raises new hypotheses about the interaction between developing executive control (specifically, increasing resistance to interference of irrelevant information) and increasing conceptual knowledge in accounting for the development of conceptual flexibility. Categorical flexibility in children: Distinguishing response flexibility from conceptual flexibility The protracted development of taxonomic representations Categorization is considered as a fundamental component of cognition and as a highly powerful tool to organize the otherwise chaotic world with limited processing capacities; it is often restricted to taxonomic categorization. Yet, other forms (perceptual or thematic) of categorization do exist and adaptive categorization can imply switching between these different forms depending on one's current goal. A ball has to be considered as a round thing if the point is to pack everything before moving to a new house, but as a thing that "goes with" a tee shirt and sport shoes (thematic grouping) if the goal is to play a football game (Barsalou, 1983). Although, recent empirical research (see below) suggests that early on, children can sort objects on a thematic or taxonomic basis, little is known about the development of categorical flexibility per se ,i.e. the ability to switch between different categorical groupings of the same set of elements. Moreover, the extent to which switching between categorical responses (i.e. sorts) reflects the actual activation of diverse types of categorical representations is an entirely open question. According to recent empirical studies, several ways of categorizing objects are available from the preschool period and even in infancy (