Psychosocial interventions are attractive options for treating chronic low back pain, and many approaches now have strong support for efficacy. However, few empirical data address whether psychosocial pain treatments work because of mechanisms specified by theory, and thus investigators know very little about HOW our treatments work. It may be that different treatments work via distinct pathways that are specific to a given treatment (single effect model), or it may be that different treatments work to the extent they all operate via key mechanisms that they share (additive effects model). Examination of specific and/or shared effects on outcomes of mechanisms will provide theoretical and empirical rationale for enhancing procedures and techniques most closely linked to strong outcomes and incorporating them into future interventions, while limiting the use of others that may be revealed as inert.