Familiarity is an elusive concept, capturing what we know intimately and what we only recognize from having seen before. This article aims to disambiguate these interpretations by proposing a sociological conceptualization of familiarity as a dynamic relationship to the world that develops over time and through experience and that allows one to progressively disattend from what appears as "usual." Focusing on how urban environments and their human entities become familiar and stop being familiar, I propose that familiarity be thought of as an ongoing relational and interactional achievement, allowing us to focus on our daily activities while relying on a practical knowledge of our surroundings. The conceptualization process unfolds via five questions: What is familiarity? Where does it come from? What threatens it? What does it produce? How can it be operationalized and studied empirically?