This paper explores the use of theory in longitudinal qualitative research, an approach to research which explores lived experiences as they unfold. The authors illustrate how the complexity of conducting qualitative research through time drives an understanding and use of theory that differs from other research approaches. Longitudinal qualitative research considers time as fluid, subjective, and unbounded--in contrast to the more common taken-for-granted understanding of time as fixed, objective, and linear. Furthermore, longitudinal qualitative research is predicated on a premise of trust in the context of enduring research relationships. Therefore, while subject-matter theories used to investigate topics of interest to health professions educators may be useful frameworks for other types of research, longitudinal qualitative research needs theories that accommodate the myriad of changes in lived experiences through time. The authors share their decade-long, longitudinal qualitative research story, highlighting their decision points and insights. In doing so, they foreground issues such as time as fluid as an important contribution to health professions education literature.