Human memory is not like a videotape that conserves faithful records of events. Rather, memories are reconstructed, and to this end people engage in a fallible process of source monitoring. When source monitoring, people judge whether their mental content constitutes a memory by considering its characteristics. For instance, vivid, plausible, familiar mental images are more likely to be memories than are mental images that possess none of these cues. Such cues—derived from a subjective inspection of mental content—are here termed internal evidence of an event’s occurrence. Contrastingly, recent research has examined the capacity of external evidence—obtained via perception rather than introspection—to promote distortions. This thesis explores the effects of external evidence on autobiographical beliefs and memories, in terms of the underlying cognitive mechanisms, the conditions under which distortions can occur, and their behavioural consequences. Experiments 1 and 2 demonstrate the capacity of external evidence to distort people’s beliefs and memories of their recent experiences, even those that are memorable, and even when people are warned about false evidence. These experiments provide preliminary data on causal mechanisms underlying the distortions, explored more directly in Experiment 3. The findings of Experiments 1-3 have numerous practical applications, particularly with respect to the functions of legal evidence. Experiments 4-5 ask whether seeing fabricated evidence might elicit false confessions from innocent suspects. The experiments show extremely high levels of compliance, and demonstrate that seeing false evidence can promote internalisation of guilt, and confabulation. Alongside discussion of theoretical implications of Experiments 1-5, the findings are used to amend Mazzoni and Kirsch’s (2002) model of autobiographical belief and memory. The amended model captures the effects of external evidence on source monitoring, and thus provides a better account of autobiographical memory processes. This thesis, in sum, helps us to understand the roles of external evidence in memory construction.