Many types of online communities rely on volunteer moderators to manage the community and maintain behavioral standards. While prior work has shown that community moderators often develop a deep understanding of the goals of their moderation context and sophisticated processes for managing disruptions,less is known about the processes through which moderators develop this knowledge. In this paper, we leverage Donald Schön's concept of reflective practice as a lens for exploring how community moderators develop the 'knowledge-in-action' that they use to perform their work. Drawing on interviews with 18 Twitch moderators, we conceptualize moderators as reflective practitioners, iteratively encountering novel situations and adjusting their practices and mental models. Our findings provide detailed insight into how community moderators reflect-in-action, re-evaluating in real-time their mental models of viewer intent and community goals, and reflect-on-action, conducting post hoc assessments of individual incidents and long-term changes to adjust their practice over time. Moderators working in teams reveal specific aspects of reflection facilitated by cooperative discussion, which we call 'groupwise reflective practice'. By identifying community moderation as a form of reflective practice, we can leverage insights gained from studying practitioners in other fields,providing theoretical and practical implications for the study and support of community moderation.