Preschoolers have long been thought to be unable to coordinate multiple representations for a single object. This paper presents recent empirical data that, in contrast, highlight a major increase in cognitive flexibility in children between 3 and 5. It then provides a critical review of the competing theoretical proposals regarding the processes involved in flexible behaviours. These theoretical proposals respectively emphasize the role of cognitive complexity, the graded nature of representations, inhibition, object redescription, and negative priming, in accounting for the development of cognitive flexibility. This review leads to new suggestions for future research that should overcome two main limitations of the current approaches: (a) the almost exclusive focalization on perseveration as the only display of a deficit in flexibility, and (b) the consideration of the executive aspects only, ignoring the role of conceptual understanding. Finally, the question of the transition mechanisms underlying the developmental changes observed in flexibility is raised in conclusion.