Many rehabilitation treatments are difficult to define, resulting in a lack of clarity about their essential active ingredients. Treatment theories can narrow the scope of possible active ingredients, by clearly specifying how the treatment is believed to act. Efficacy studies of theory-defined treatments assess their clinical value, but also advance the science underlying the theory. In addition, treatment theories shape inclusion and exclusion criteria by suggesting what types of patients may benefit, and shape outcome measures by suggesting where the treatment impact should be seen. Finally, treatment theories can assist an investigator in the selection of an optimal study design.