Employees' proactive behaviour is increasingly important for organizations seeking to adapt in uncertain economic environments. This study examined the link between leadership and proactive behaviour. We differentiated between organizational leadership and team leadership and proposed that transformational leadership by team leaders would enhance commitment to the team, which would predict team member proactivity. In contrast, transformational leadership by leaders of the organization would enhance commitment to the organization, which we expected to predict organization member proactivity. Transformational leadership on both levels was expected to increase employees' role-breadth self-efficacy, the confidence necessary to engage in proactive behaviour. Our results demonstrate the importance of leadership as an antecedent of proactive work behaviour and suggest that leadership at different levels influences proactivity via different mediators. Transformational team leaders seem to facilitate proactivity by increasing employees' confidence to initiate change. Transformational organizational leaders on the other hand increase proactivity by enhancing employees' commitment to the organization.