In this chapter I introduce a distinction between two research types, marker studies and transition studies, that I find useful in developing and categorizing hypothetical learning trajectories (hlts). I stress the latter type as one closely related with the third element in Simon’s (1995) definition of hlt. To further depict the linkage between hlt and transition studies, I discuss the starting points and development of hlts, including a model of learning as cognitive change – reflection on activity-effect relationship. I follow those sections with a description of four ways in which I find hlts that specify conceptual transitions to be useful.