Optimal competence is vital in safety-critical professions. To optimize (refresher) training, insight into the process of competence decay and influencing factors on competence decay is crucial. Although retention of knowledge and simple skills has been studied for many years, literature on retention of complex skills remains limited. This literature review focuses on the factors influencing retention that have been studied empirically since 2010 in the context of safety-critical professions. Furthermore, the nature of competence retention curves over time was studied. Methods. The initial search yielded 882 articles, which were scanned systematically according to inclusion criteria. The main inclusion criteria were whether refresher training or longitudinal designs were used to study competence retention. The final set of articles included in this review contains 40 articles. Results. 39 studies were carried out in the medical domain, only one article focused on aviation. The majority of studies showed that competence decreases over time. Important factors related to retention were: (a) the quality of initial training, (b) practice or refreshers, (c) personal factors, and (d) task complexity. In 23 articles enough data was provided to reconstruct and compare skill decay curves. Novices who did not practice the skill after their initial training showed faster skill decay than novices who were allowed to practice, and participants with mixed or little experience. Recommendations. More empirical research should be conducted to determine the optimal refresher interval and to study the effects of individual differences, especially regarding competence retention of complex skills outside the medical domain.