Background/Context: Based on their case studies of preparation for professional practice in the clergy, teaching, and clinical psychology, Grossman and colleagues (2009) identified three key concepts for analyzing and comparing practice in professional education--representations, decomposition, and approximations--to support professional educators in learning from each other's practice. In this special issue, two teams of teacher educators (Kucan & Palincsar, and Boerst, Sleep, Ball, & Bass) put these concepts to work in representing their practice of preparing novice teachers to lead discussions with their students. Purpose/Objective/Research Questions/Focus of Study: This analytic essay presents an argument for the importance of (a) adding a fourth key concept to the Grossman et al. framework--conceptions of quality--and (b) using these four concepts to trace novices' learning opportunities as they unfold over time in order to serve the goal of facilitating instructive comparisons in professional education. Research Design: In this analytic essay, I analyze the three articles to examine how conceptions of quality are already entailed in the characterizations of practice. My analysis focuses on the kinds of criteria or "qualities" that are foregrounded; the grain size of practice to which the conception of quality is applied; and the ways in which variations in criteria--what counts as more or less advanced--are represented. I then contrast the sequence of learning opportunities and assessments described in the articles on discussion leading in terms of these four concepts. Conclusions/Recommendations: Even instructional practices that appeared quite similar when described through the lenses of approximations, decomposition, and representations looked quite different when conceptions of quality and learning opportunities and assessments were traced over time. Representing these "learning trajectories"--which entail an understanding of the evolving dialectical relationships between learning opportunities and (at least intended) learning outcomes--seems essential to understanding and learning from the teaching practice.