With the advent of apps, other software and wearable devices designed to enable people to easily monitor and measure details about their bodies, much attention has been paid to the phenomenon of health self-tracking. In this article, findings from a study involving interviews with 40 Australian self-trackers are discussed and analysed from a feminist new materialist perspective, focusing on relational dimensions, affective forces and agential capacities. Analysis of their accounts identified several major agential capacities generated by self-tracking health and illness, including achieving knowledge, awareness and problem-solving; taking control; and feeling better. Affective forces were strongly evident in the ways the participants talked about their practices and rationales for health self-tracking, including the pleasure and satisfaction they experienced, as well as the demoralising or burdensome elements they described. Relational dimensions included interpersonal and biographical contexts as well as enactments of embodied and technological sensing and recording.