Musculoskeletal pain is a common condition among children and adolescents. In general, and more specifically in Brazilian-Portuguese, there is a lack of questionnaires – and the summarisation of the available ones – to measure the presence and impact of musculoskeletal pain, disability, and its associated factors in children and adolescents. There is a lack of longitudinal studies (especially on prognosis) conducted with children and adolescents with musculoskeletal pain, especially in low- and middle-income countries. Also, we do not know the methodological and the reporting quality of systematic reviews on conservative non-pharmacological interventions in children and adolescents with musculoskeletal pain. Therefore, the overall aim of this thesis is to describe, and summarise the available instruments to measure disability in children and adolescents with musculoskeletal pain; translate, cross-culturally adapt, and test the measurement properties of relevant instruments for children and adolescents; to assess the development, the course, and the cost of musculoskeletal pain in children and adolescents; and measure the quality of systematic reviews of interventions for musculoskeletal pain in children and adolescents. The first specific aim of this thesis (Chapter 2) was to describe the available Patient reported outcome measures (PROMs); critically appraise measurement properties; and grade the level of evidence. We included 132 studies measuring 31 PROMs. The PROM development and content validity range from very low certainty of the evidence of inconsistent to moderate certainty of evidence of sufficient PROM development and content validity. The other measurement properties range from very low certainty of evidence of insufficient to high certainty of evidence of sufficient measurement properties. To make available a PROM to measure constructs related to musculoskeletal pain in children and adolescents, the second aim of this thesis (Chapter 3) was to translate, cross culturally adapt (to Brazilian-Portuguese), and test the measurement properties (structural validity, reliability, measurement error, construct validity) of the Presence and Impact of Pain in Kids (PIP-Kids) questionnaire in a population of Brazilian children and adolescents. We recruited 656 children and adolescents from public and private schools with and without musculoskeletal pain. The process of translation and cross-cultural adaptation showed that the instrument did not need any change. Structural validity confirmed two domains (presence and impact of pain). Reliability ranges from 0.20 to 0.68 by the Kappa Coefficient. Measurement error by the percentage of agreement ranges from 60.2 to 92%. Construct validity was confirmed at 80.5% in accordance with prior hypotheses. To make available another instrument to measure constructs related to pain in children and adolescents, the third aim of this thesis (Chapter 4) was to translate, cross-cultural adapt (to Brazilian-Portuguese), and test measurement properties (reliability, internal consistency, measurement error, and construct validity) of the Psychosomatic Questionnaire for Children and Adolescents (PQCA) in a population of Brazilian children and adolescents with musculoskeletal pain. We recruited 107 children and adolescents with disabling musculoskeletal pain from public and private schools. The Psychosomatic Questionnaire for Children and Adolescents (PQCA) showed an adequate translation and cross-cultural adaptation process and no ceiling and floor effects. Internal consistency was 0.69 by Cronbach’s Alpha. Reliability was 0.75 (95% CI, 0.64 to 0.84) by Intraclass Correlation Coefficient. Measurement error by Smallest Detectable Change was 6.5 points out of 18 points. We observed an adequate construct validity with correlation in accordance with our prior hypothesis. With the aim of understanding disabling musculoskeletal pain over time and developing a prediction model in children and adolescents, the fourth specific objective of this thesis was to describe the course of disabling musculoskeletal pain in children and adolescents and to develop a prediction model to identify children and adolescents more likely to recover from disabling musculoskeletal pain within 18 months (Chapter 5). We recruited 694 children and adolescents with disabling musculoskeletal pain from public and private schools in Brazil. The course of disabling musculoskeletal pain was measured by pain recovery, with frequencies and percentages over time. We developed a theoretical predictive model to support our prediction model for recovery. The dependent variable was the recovery from disabling musculoskeletal pain, and the candidate predictor variables were psychosomatic symptoms, sleep quality, quality of life, single or multiple pain sites, sex, age, and socioeconomic status. The model was measured through a logistic mixed model. The recovery from disabling musculoskeletal pain was 85.9%. The model showed statistical significance for quality of life and age, with low to significant model performance values. To investigate the epidemiology of back pain in children and adolescents, the fifth aim of this thesis (Chapter 6) was to estimate the prevalence, incidence and prognosis of back pain in children and adolescents. We recruited 615 children and adolescents from public and private schools with and without back pain. Two to three in every 10 children and adolescents reported back pain in the last month. Of children and adolescents with back pain, almost all recover in 12 months, but recurrence seems to be common. New cases of back pain were reported by three to four in every 10 children and adolescents in 12 months. With the purpose of understanding how costly musculoskeletal pain is in children and adolescents, the sixth specific objective of this thesis is to estimate the cost of disabling musculoskeletal pain in children and adolescents from the perspective of healthcare and society (Chapter 7). We recruited 237 children and adolescents with disabling musculoskeletal pain from public and private schools. We followed these children and adolescents for 12 months. In the follow-up assessments, questions regarding healthcare costs (e.g., medications, medical consultations), productivity loss costs, and family/patient costs (e.g., out-of-pocket expenses, self-purchased medications) were conducted. Healthcare costs (health insurers and public) were $29.58 per child and adolescent annually. Healthcare costs (private) were $103.13 per child and adolescent annually. Productivity loss costs for parents and/or caregivers were $29.25 per child and adolescent annually. Family/patient costs were $15.65 per child and adolescent annually. And societal costs were $177.62 per child and adolescent annually. The estimated budgetary impact of disabling musculoskeletal pain was approximately $1 billion annually. There were no sociodemographic or health differences between children and adolescents with high and low costs from a societal perspective. And the last aim of this thesis (Chapter 8) was to evaluate the methodological quality and the reporting quality of systematic reviews on conservative non-pharmacological pain management of children and adolescents with musculoskeletal pain. We used A MeaSurement Tool to Assess systematic Reviews 2 (AMSTAR 2) to assess the methodological quality and the Preferred Reporting Items for Systematic reviews and Meta-Analysis (PRISMA) to assess reporting quality. We found 17 systematic reviews on conservative non-pharmacological pain management for children and adolescents with musculoskeletal pain. The methodological quality of the included systematic reviews ranges from ‘very low’ to ‘high’. The reporting quality was generally poor, with the item with the worst description being the risk of bias across studies and the item with the best description being the study characteristics. Through the studies conducted in this thesis, we have gained a comprehensive understanding of the overall scenario of musculoskeletal pain in children and adolescents in a middle-income country like Brazil. This understanding encompasses aspects such as prevalence, incidence, the course of pain, and its associated costs. Additionally, we have identified instruments for assessing disability in children and adolescents with musculoskeletal pain, and we have made available two instruments in the Brazilian Portuguese language. Furthermore, we have developed a prediction model with the intention of promoting a deeper understanding of this condition among children and adolescents. This model aims to help us identify which children and adolescents are more likely to recover from this type of pain. Chapter 9 of this thesis provides the final considerations on the topics and chapters discussed throughout the thesis.