Adults can mentally relive experiences from their past and anticipate possible future events, a process called mental time travel (MTT). Recently, several theorists have argued that the ability to mentally travel through time may not emerge until 3- to 5-years of age. This proposal is based on evidence from a wide range of research, including investigations into children’s recall, planning and differentiation of the times of events in the past and the future. However, as yet there has been no dedicated effort to find out how and when MTT develops. The current series of studies brought together a wide range of resources with the aim of designing a series of novel paradigms to measure aspects of MTT development between 3- and 5-years of age. The first of these novel approaches asked children to report events that occurred to them “yesterday” and would occur to them “tomorrow”, revealing that by 4- to 5-years of age most children could accurately do so. Another series of studies examined children’s ability to anticipate a different, future situation, finding that only by 4- to 5-years did children’s behaviour reflect differences in an anticipated future environment. Subsequent studies focusing on children’s discrimination of past and future also suggested that by 4- to 5-years children could distinguish the different causal impact of past and future events on the present. Another task revealed that during the preschool years children become better at differentiating the times of events from throughout their own lifespan. These new data provide support for the claim that the ability to mentally travel into one’s own past and future emerges during the preschool years. This thesis describes the first directed investigation into the development of MTT as a whole, bringing together much of the empirical and theoretical literature for the first time. The tasks designed are new approaches to investigating MTT and represent a starting point for future research. This thesis also introduces and discusses theories for the development of MTT, aiming to stimulate discussion not just of when it emerges, but how, and what processes may underlie the transition.