Activity tracking apps –such as Fitbit or Nike+ Running– are positioned to enhance people’s motivation towards physical activity and healthy behavior. Though various ‘motivational’ features are incorporated, the effectiveness of these apps is often mixed raising concerns about the ‘one-size-fits-all‘ applicability. To get a better understanding about the nature of using activity tracking apps, this paper employs the lens of affordances and identifies eight particular salient affordances in activity tracking: Self-Monitoring, Performance Analysis, Exercise Guidance, Rewards, Social Comparison, Watching Others, Social Recognition, and Self-Presentation. Moreover, this study develops a corresponding measurement instrument evaluated using q-sort methodology. Avenues for future research are highlighted involving the set of affordances and their instruments and practical implications are given.