Background: There is a paucity of empirical and theoretical literature on reality and realism in simulation-based learning. Methods: This article makes an original contribution to the body of literature by using the theoretical conceptualizations of reality described by Dieckmann et al. in simulation and by the fantasy author and scholar J.R.R.Tolkien to challenge and develop our understanding of reality in simulation.This article reports a qualitative research study that reveals the perceptions of realism in simulation-based medical education. Results: A significant finding was the importance participants placed on realism in their motivation to participate in simulation. Participants descriptions of realism were consistent with the domains of physical, semantic and phenomenological realism and played an important role in their intention to participate in simulation-based learning. The data also revealed that while lapses in physical realism were tolerated, lapses in semantic realism were very poorly tolerated.In addition, when there was inconsistency within the secondary reality as described by Tolkien, this resulted in a breakdown of suspension of disbelief.Conclusions: A deeper understanding of these factors will inform course designers as they consider the processes of simulation-based learning and as they seek robust evidence for its effectiveness. Developing understanding of realism for medical simulation can help course designers and teachers make best use of resources by focussing on the domains of realism that have most impact on learners.