Based on a three-year-long ethnography of the hobby of model rocketry, I present a practice-centered theory of interest relationships--that is, the pattern of long-term, self-motivated engagement in open-ended practices that has been theorized under the concept of "individual interests". In contrast to extant theories of individual interests, in which persistent engagement is pegged to a topic-specific relationship (e.g., a model rocketeer has an interest in the topic of rocketry, broadly conceived), I propose that persistence in a practice of interest is best understood in terms of what I call "lines of practice". A line of practice is a distinctive, recurrent pattern of "long-term" engagement in a person's practice participation. Any line of practice entails a set of closely interrelated activities that are defined by two structural elements: "preferences" and "conditions of practice". Preferences refer to the deep, long-term goals, values, and beliefs that a person develops in the practice, whereas conditions of practice refer to the constraints and affordances impinging on the person's practice (e.g., socioeconomic status and the norms of practice sites). A line of practice can thus be seen in the distinctive ways that a person's preferences are attuned, over the long haul, to specific conditions of practice, and how preferences cluster to form any single line. I show how persistent engagement can be understood in terms of the birth, death, and changes to a person's lines of practice and describe how this formulation broadens current theories of individual interests. (Contains 3 figures and 3 footnotes.)