Reflection is a complex concept in medical education research. No consensus exists on what reflection exactly entails; thus far, cross-comparing empirical findings has not resulted in definite evidence on how to foster reflection. The concept is as slippery as soap. This leaves the research field with the question, 'how can research approach the conceptual indeterminacy of reflection to produce knowledge?'. The authors conducted a critical narrative umbrella review of research on reflection in medical education. Forty-seven review studies on reflection research from 2000 onwards were reviewed. The authors used the foundational literature on reflection from Dewey and Schön as an analytical lens to identify and critically juxtapose common approaches in reflection research that tackle the conceptual complexity. Research on reflection must deal with the paradox that every conceptualization of reflection is either too sharp or too broad because it is entrenched in practice. The key to conceptualizing reflection lies in its use and purpose, which can be provided by in situ research of reflective practices.