The last five years has seen an explosion of interest in research on the return of extinguished conditioned behaviour in humans. This interest has resulted from the development of theoretical models of the phenomenon from non-human animal research and the potential application that the research has to explaining relapse following extinction-based treatments for psychological disorders. The recent research conducted with human participants is reviewed. The main return of conditioned behaviour phenomena are renewal, reinstatement, spontaneous recovery, and reacquisition, although recent human research has tended to focus on only the first two. Human research has employed three main paradigms in the laboratory: fear conditioning procedures, a conditioned suppression task, and causal learning tasks. The basic effects of renewal, reinstatement, and spontaneous recovery have been demonstrated in these tasks and in psychophysiological, behavioural, and subjective measures. However, research has also yielded some inconsistencies among measures as well as differences between the results obtained with human participants and those obtained with non-human animals. There may be some unique factors that mediate the return of extinguished conditioned behaviour in humans. Clinical studies on anxiety and substance dependence have also confirmed that renewal, reinstatement, and spontaneous recovery are potentially important mechanisms that may underlie relapse following extinction-based treatments. There is a clear need for future research to understand the psychological mechanisms behind the return of extinguished conditioned behaviour in humans and to develop ways to reduce its occurrence in clinical treatment.