Mass casualty events (MCE) strain available health-care resources requiring extraordinary measures. Simulated exercises are used to improve preparedness. We sought to identify learning points and common themes arising from such exercises in literature. Reporting of action points to improve response plans were investigated. Type of exercises, environments, and departments were also explored. We systematically searched 3 databases and applied our eligibility criteria. Inclusion criteria were in-situ MCE simulations of clinical response to traumatic MCEs, including scene management, prehospital care, and in hospital care. Exclusion criteria were nonmedical response, infectious outbreaks, training courses with self-selecting participants, simulations assessing mechanical tools, and mathematical modeling. A total of 6883 titles were identified and screened. Eighty-three studies were read in full. Twenty-two articles were included. We identified numerous learning points, which were collated and categorized into 11 themes. Fifty-nine percent of the papers reported actions that would be or had been implemented. MCE simulation exercises have been found to improve familiarity and confidence among participants. The 11 themes identified from published exercises overlap with areas of improvement from real events. MCE simulations in the literature appear to focus on carrying out the exercise itself rather than learning points possibly missing opportunities to improve response plans.