Chronic postsurgical pain (CPSP) has been recognized as a major health concern across the lifespan. Adolescents undergoing invasive musculoskeletal surgeries are particularly at risk for CPSP, which occurs in about 20% of youth after surgery. CPSP is associated with significant functional disability and reduced physical and psychosocial health-related quality of life. Chronic pain in adolescence also places individuals at risk for chronic pain in adulthood, as well as risk for substance use disorder, generating potential lifelong consequences on functioning, productivity, and quality of life. In our own preliminary studies in a small sample of adolescents having major surgery, we demonstrated that (1) youth who develop CPSP may follow distinct recovery patterns that emerge within the first 2 weeks following surgery, and (2) baseline psychosocial risk factors predict development of CPSP. However, further studies are needed to understand recovery during the initial weeks following surgery when acute postsurgical pain begins to transition to CPSP. Although there is indication from prior studies that psychosocial factors may play a role in CPSP, there has been limited data on further biopsychosocial mechanisms that influence the transition from acute to chronic pain after musculoskeletal surgery in adolescents. These gaps in knowledge have limited the development and implementation of perioperative interventions targeted at the mechanisms of the transition from acute to CPSP to positively alter the trajectory of postsurgical recovery. Thus, this project aims to 1) develop valid and reliable acute recovery indices using short-term trajectories of pain, sleep quality, mood, and physical function over the first 30 days following spinal fusion surgery, and 2) determine the psychosocial and psychophysical mechanisms contributing to the transition from acute to chronic postsurgical pain. To address these aims, we propose a 2-site prospective longitudinal cohort study in 160 youth aged 10 to 18 years undergoing spinal fusion surgery, and their parents. Prospective assessments of pain, health, and functional outcomes will be collected before surgery and at three time points during the 6 months after surgery. Immediately after surgery, adolescents will complete daily monitoring of pain, sleep quality, mood, and physical function in the first 30 days following hospital discharge using ecological momentary assessment. These data will be used to develop valid and reliable acute recovery indices that predict CPSP at 3 and 6 months post-surgery. We will measure two potential sets of mechanisms underlying the transition from acute to CPSP, psychosocial variables and laboratory-based psychophysical pain responses, before surgery and at 8-weeks post-surgery in order to determine the temporal influence on subsequent development of CPSP. This study will increase understanding of the transition from acute to chronic postsurgical pain and the causal mechanisms involved. ...