The field of rehabilitation remains captive to the black-box problem: our inability to characterize treatments in a systematic fashion across diagnoses, settings, and disciplines, so as to identify and disseminate the active ingredients of those treatments. In this article, we describe the Rehabilitation Treatment Specification System (RTSS), by which any treatment employed in rehabilitation may be characterized, and ultimately classified according to shared properties, via the 3 elements of treatment theory: targets, ingredients, and (hypothesized) mechanisms of action. We discuss important concepts in the RTSS such as the distinction between treatments and treatment components, which consist of 1 target and its associated ingredients; and the distinction between targets, which are the direct effects of treatment, and aims, which are downstream or distal effects. The RTSS includes 3 groups of mutually exclusive treatment components: Organ Functions, Skills and Habits, and Representations. The last of these comprises not only thoughts and feelings, but also internal representations underlying volitional action; the RTSS addresses the concept of volition (effort) as a critical element for many rehabilitation treatments. We have developed an algorithm for treatment specification which is illustrated and described in brief. The RTSS stands to benefit the field in numerous ways by supplying a coherent, theory-based framework encompassing all rehabilitation treatments. Using a common framework, researchers will be able to test systematically the effects of specific ingredients on specific targets; and their work will be more readily replicated and translated into clinical practice.