Abstract : The main objective of this thesis was to investigate the role and impact of stereotypes on the physical activity levels of individuals with cancer. The first section defends the hypothesis that the beliefs of people with cancer about physical activity reflect the internalisation of stereotypes, which affect their behaviors regarding physical exercise. The first two studies of this thesis identify five categories of beliefs about physical activity and show that four of them are psychological barriers to activity, and develop and validate a scale to measure the stereotypes about cancer and physical activity. The results of the third study indicate that these stereotypes are related to the level of physical activity of cancer patients. The fourth study presents evidence of the positive impact of the exerciser status in shaping impressions in the general population about people with cancer. The secondary objective of this dissertation was to identify the conditions needed to change the beliefs about and the motivation for physical activity in the cancer population. Two experimental studies examined the effects of messages promoting physical activity on sedentary patients. Study 5 showed that informational messages resulted in more favorable beliefs, whereas narrative messages were more appropriate to stimulate motivation. Study 6 confirmed the hypothesis that a narrative message from an intrinsically motivated peer was more efficient in improving the motivation to exercise in sedentary patients than the same type of message from an extrinsically motivated peer.