Self-monitoring activities are increasingly parts of people's everyday lives. Some of these measurements are performed voluntary rather than being referred by a physician, and conducted because of either a preventive health interest, or to better understand the body and its functions (so-called 'Quantified Self'). In this article, we explore socio-technical complexities that may occur when introducing preventive health-measurement technologies into older adults' daily routines and everyday lives. In particular, the original study investigated blood pressure (BP) measurement in nonclinical settings, to understand existing challenges, and uncover opportunities for self-monitoring technologies to support preventive healthcare activities among older adults. From our study, several aspects emerged that are important to consider when designing preventive self-monitoring technology, such as the complexity of guidelines for selfmeasuring, the importance of interpretation, understanding and health awareness, sharing self-monitoring information for prevention, various motivational factors, the role of the doctor in prevention, the home as a distributed information space, and so on. An awareness of these aspects may help designers to develop better support tools for people's preventive self-monitoring needs, compared to existing solutions. Supporting the active and informed individual may improve their self-care, awareness, and implementation of preventive care. Based on our study, we reflect on the findings to illustrate how these aspects may also inform both people engaged in Quantified Self activities and designers alike, and the tools and approaches that have sprung from the so-called 'Quantified Self' movement.