This dissertation uses the rise of wearable fitness tracking as a lens through which to examine the predominance of quantification in everyday life. With over a third of U.S. adults owning a tracker, the increased use of body-surveilling technologies provides an opportunity to investigate some central sociological questions. In this project, I ask why individuals track their health behaviors with technology and how we may understand this behavior in the context of medical and corporate interests. I further ask how people think about the privacy implications of their use and what concerns they have about data collection and sharing. I bring together literature in the social construction of health, privacy, surveillance, and the culture of measurement to situate the case. In the analysis, I first review data about the industry to lay down a groundwork of the institutional and medical context for trackers, but focus the analysis on 55 semi-structured qualitative interviews I conducted with tracker users. This allows for attention on the experiences of people who use trackers as they go about their daily routines. Key informant interviews with medical professionals contextualize this case. In a review of the quantitative research, I find that the commercial value of trackers lies in data they generate rather than their hardware. This connects to the qualitative interview analysis, which focuses on two primary sets of issues: goals and habits and surveillance and privacy. I find that respondents identify mostly internal motivations for tracking but experience overt and subtle nudges from their technology. I further assert that technology itself has changed the nature of what self-measurement is possible and seen as normal, thus contributing to scholarship on the culture of measurement. I also find that attitudes about data privacy vary depending on context and expressed trust in institutions. In the overall analysis, I reaffirm the growing role of quantification in society and go on to introduce the concept of the quotidian quantifier as the personification of the modern individual who engages in tracking as they enact their daily routines. This concept elucidates the importance of attention to the mundane in social science research.