Rehabilitation is in need of an organized system or taxonomy for classifying treatments to aid in research, practice, training, and interdisciplinary communication. In this article, we describe a work-in-progress effort to create a rehabilitation treatment taxonomy (RTT) for classifying rehabilitation interventions by the underlying treatment theories that explain their effects. In the RTT, treatments are grouped together according to their targets, or measurable aspects of functioning they are intended to change; ingredients, or measurable clinician decisions and behaviors responsible for effecting changes; and the hypothesized mechanisms of action by which ingredients are transformed into changes in the target. Four treatment groupings are proposed: structural tissue properties, organ functions, skilled performances, and cognitive/affective representations, which are similar in the types of targets addressed, ingredients used, and mechanisms of action that account for change. The typical ingredients and examples of clinical treatments associated with each of these groupings are explored, and the challenges of further subdivision are discussed. Although a Linnaean hierarchical tree structure was envisioned at the outset of work on the RTT, further development may necessitate a model with less rigid boundaries between classification groups, and/or a matrix-like structure for organizing active ingredients along selected continua, to allow for both qualitative and quantitative variations of importance to treatment effects.