Chronic musculoskeletal pain (CP) is a major public health concern. A number of psychosocial treatments have emerged in recent decades to help address this problem. These interventions have been shown to be efficacious when compared to largely inert control conditions; however, recent meta-analyses indicate that most of these treatments are characterized by modest effects on primary outcomes. This is a critical shortcoming of these otherwise promising approaches. Rather than attempting to boost efficacy only by developing new and hopefully more powerful interventions, we can also look within our already proven treatments for ways to enhance the magnitude of treatment effects. One strategy is to increase our understanding of treatment mediators. Studies of mediators are needed that directly compare different treatments with each other to determine which mediators are treatment-specific, which are shared across treatments, and which contribute the most to clinical outcomes. The findings from such research could be used to inform adaptations to existing treatment that enhance their benefits. A second strategy for increasing the beneficial effects of existing treatments is to identify the patient characteristics that moderate treatment responses. Research is needed that is guided by theoretical models and that tests moderators across multiple treatments. Identifying subgroups of patients more likely to respond to one or another treatment can advance precision medicine by informing a priori patient-treatment matches that can optimize treatment effects. We will compare the mechanisms and moderators of Cognitive-Behavioral Therapy (CBT), Acceptance and Commitment Therapy (ACT), and Emotional Awareness and Expression Therapy (EAET). Examining the mediators and moderators of these treatments holds great potential for advancing treatment development and enhancing treatment efficacy. Adults with chronic spinal (axial) pain (N=460) will be randomly assigned to CBT, ACT, EAET and to treatment-as-usual (TAU). Aim 1 is to identify specific and shared mediators across treatments. Aim 2 is to identify moderators across treatments. This project can increase the effects of our psychosocial chronic pain treatments by identifying the most powerful treatment mechanisms – specific and shared -- and revealing for whom the mediator outcome pathways are strongest. Via increased understanding of mediator and moderators, more effective pain treatment approaches can be developed, tested, and implemented.