From sleep and energy use to exercise and health, consumers have access to more information about their behavior than ever before. The appeal of personal quantification seems clear. By better understanding their behavior, consumers can make the necessary changes to live happier, healthier lives. But might the new tools consumers are using—quantifying life— rob them of some of the benefits of engaging in those activities? Six experiments demonstrate that while measurement increases how much of an activity people do (e.g., walk or read more), it can simultaneously reduce how much people enjoy those activities. This occurs because measurement can undermine intrinsic motivation. By drawing attention to output, measurement can make enjoyable activities feel more like work, which reduces their enjoyment. As a result, measurement can decrease continued engagement in the activity and subjective well-being. Even in the absence of explicit external incentives, measurement itself can thus have similar effects. The findings have implications for measurement’s use, as well as for the psychology of external incentives and intrinsic motivation.