One strategy for addressing the persistent challenge of advancing empirically based social work practice is to engage practitioners in the formulation of empirically developed practice guidelines. Although this approach is promising, there has been relatively little work to guide practitioners, and perhaps consequently, few examples of practitioner-developed guidelines exist. Moreover, available treatment models do not address most social work practice concerns. One such concern involves sibling aggression, which is exceptionally widespread with known deleterious consequences, but it has received scant attention in the intervention literature. This article illustrates a practitioner's development of a task-centered sibling aggression treatment model using the design and development research paradigm, a methodology for systematically formulating, testing, and refining practice guidelines. It also introduces a promising sibling aggression treatment model. (Contains 6 figures.)