In human research, return of fear (ROF) phenomena and reinstatement in particular, have only begun to be studied a decade ago and are recently more widely used e.g. as outcome measures for fear/extinction memory manipulations (e.g. reconsolidation). As reinstatement research in humans is still in its infancy, providing an overview of its stability and boundary conditions and summarizing methodological challenges is timely to foster fruitful future research. As a translational endeavor, clarifying the circumstances under which (experimental) reinstatement
occurs may offer a first step towards understanding relapse as a clinical phenomenon and pave the way for the development of new pharmacological or behavioral ways to prevent ROF. The current state of research does not yet allow pinpointing these circumstances in detail and we hope that this review will aid the research field to advance in this direction. As an introduction, we begin with a synopsis of rodent work on reinstatement and theories that have been proposed to explain the findings. The review however mainly focuses on reinstatement in humans. We first (1) describe details and variations of the experimental setup in reinstatement studies in humans and give a general overview over results. We continue with (2) a compilation of possible experimental boundary conditions and end (3) with the role of individual differences and behavioral and/or pharmacological manipulations. Furthermore, we (4) compile important methodological and design details on the published studies in humans and end with (5) open research questions and some important methodological and design recommendations as a guide for future research.