In an era of increasing interest in self-monitoring technologies to improve population health, this article considers how participants in a public health trial engaged with such technologies. Exploring how their engagement sits with the logic of self-monitoring and the technology of the trial highlights that the trial's blackboxing of its objects of study obscure the deeply contextualized care practices through which such technologies "work." Attending to (self-)care and what the trial neglects offers a means of disrupting entrenched values in its objects, relations, and logics, questioning what is important and for whom through a critical anthropology of/through health technology.