ABSTRACTA wide range of wearable fitness-trackers are currently available that allow users to measure, monitor, visualize, and record numerous training metrics including moving pace, distance traveled, average heart rate, and calories burned. Using qualitative data collected through semi-structured interviews with amateur endurance athletes, this paper examines what individuals do with their wearable fitness-trackers and the data they produce. Drawing on the work of Deborah Lupton and Sarah Maslen, we take up the concepts of “data sensing” and the “more-than-human sensorium” to highlight the embodied and sensory dimensions of digital self-tracking. We argue that while much of the appeal of fitness-tracking technologies lies in their ability to generate objective readings of one’s performance, these devices do not supplant less quantifiable and more subjective ways of understanding one’s self. On the contrary, the participants in our study use the quantitative data generated by a fitness-tracker in conjunction with their own self-assessments to gain a more holistic sense of what they are experiencing during training or on race day. For many of our research participants, the fitness-tracker became a central part of their identity and daily routine. Most participants were reluctant to train without their fitness-trackers, even when not preparing for an event.