Background: Although preparation for educational activities is considered beneficial for student learning, many students do not perform preparatory assignments. This phenomenon has received little attention in the literature although it might provide medical educators with the opportunity to enhance student learning. Therefore, we explored why students prepare or not prepare. Methods: An explorative mixed methods study was performed. In a qualitative study, 24 short group interviews with medical undergraduate students (n=209) were conducted on why they prepared for skills training sessions. In a subsequent quantitative study the resulting themes were used to construct a questionnaire. The questionnaire was presented to all undergraduate medical students at Maastricht University and 847 students completed it. Scales were constructed by a combination of exploratory factor analysis, reliability analysis, and content analysis. Between-class differences in the scale scores were investigated using ANOVA. Results: The qualitative study showed that students' opinions on preparation are influenced by both personal factors, categorized as 'personal learning style', 'attitudes and beliefs', and 'planning and organization', as well as external factors, including 'preparatory advice', 'pressure, consequence, and checking of preparation', 'teacher-related motivations', and 'contents and schedule of the training sessions'. The quantitative study showed that 'the objective structured clinical examination' and 'facilitation of both understanding and memorizing the learning material', were the two most motivating items. The two most demotivating aspects were 'other students saying that preparation was not useful' and 'indistinct preparatory advices'. Factor analyses yielded three scales: 'urge to learn', 'expected difficulties', and 'lack of motivation'. Between group differences were found between the three classes on the first two scales. Conclusions: Students make an active and complex choice whether to prepare or not, based on multiple factors. Practical implications for educational practice are discussed.