Background: Although simulation-based assessment (SBA) is being implemented in numerous medical education systems, it is still rarely used for undergraduate medical students in France. Objective structured clinical examinations (OSCEs) will be integrated into the national medical curriculum in 2021. In 2016 and 2017, we created a mannequin SBA to validate medical students' technical and psychometric skills during their emergency medicine and paediatric placements. The aim of our study was to determine medical students' perceptions of SBA. Methods: We followed the grounded theory framework to conduct a qualitative study. A total of 215 students participated in either a paediatric or an emergency medicine simulation-based course with a final assessment. Among the 215 participants, we randomly selected forty students to constitute the focus groups. In the end, 30 students were interviewed. Data were coded and analysed by two independent investigators within the activity theory framework. Results: The analyses found four consensual themes. First, the students perceived that success in the SBA provided them with self-confidence and willingness to participate in their hospital placements (1). They considered SBA to have high face validity (2), and they reported changes in their practice after its implementation (3). Nevertheless, they found that SBA did not help with their final high-stakes assessments (4). They discussed three other themes without reaching consensus: stress, equity, and the structure of SBA. After an analysis with activity theory, we found that students' perceptions of SBA underlined the contradictions between two systems of training: hospital and medical. We hypothesise that a specific role and place for SBA should be defined between these two activity systems. Conclusion: The students perceived that SBA would increase self-confidence in their hospital placements and emphasise the general skills required in their future professional environment. However, they also reported that the assessment method might be biased and stressful. Our results concerning a preimplementation mannequin SBA and OSCE could provide valuable insight for new programme design and aid in improving existing programmes. Indeed, SBA seems to have a role and place between hospital placements and medical schools.