Abstract. Besides other popular strategies, such as feedback and (social) comparisons, challenges have been proposed and used to influence people's behavior towards a targeted goal. However, only very limited data on the effectiveness of such approaches and how to best design them is available yet. In this work we report the findings of a two months field study analyzing the effectiveness and perception of challenges in the context of influencing personal mobility. Individual and collaborative approaches towards challenges were studied, and specific focus was laid on what aspect makes users willing to participate in these challenges. Our findings suggest that both individual and collaborative challenges have the potential to sustain the interest of users in using behavior change support systems, that collaborative and individual challenges seem to not attract different types of users, that individual challenges in general are preferred, and that challenges are only a useful means for a subset of users. Also, ICT-competence seems to be an important aspect of being willing to participate in electronically organized challenges.