This book offers practical, evidence-based recommendations and strategies for meaningfully involving adolescents aged 10-24 years in all stages of the health research process to practitioners and professionals. The under-involvement of adolescents in health research has been attributed to a lack of awareness among researchers on how to engage adolescents. The currently available guidelines are limited in terms of the scope, content, and contexts for which the guidelines are applicable. To address this, the authors have developed a comprehensive set of guidelines on adolescent involvement in health research that are based on a systematic umbrella review of adolescent involvement in research, a rapid review of currently available guidelines on adolescent involvement, a qualitative study with researchers and adolescents from low-, middle-, and high-income countries, and participatory workshops with adolescents and researchers from 14 countries. The book is organized to provide systematic guidance on all key aspects of adolescent involvement in health research. It introduces the significance of adolescent involvement and the positive impacts of involving adolescents in health research. Following this, the authors provide guidance on the methods and levels of adolescent involvement, planning and preparation, recruitment and reimbursement practices, training for both adolescent and adult researchers, ethical considerations, and monitoring and evaluation of adolescent involvement. Each chapter provides detailed recommendations and practical strategies, ensuring that the strategies are actionable and grounded in rigorous research and stakeholder consultation. In addition, each chapter includes case studies and commonly experienced barriers along with strategies to mitigate these barriers. By offering practical insights and evidence-based strategies, A Comprehensive Guide to Adolescent Involvement in Health Research equips readers with the necessary tools to effectively engage adolescents in health research. The book serves as a valuable and comprehensive guide for researchers and practitioners involved in health research, particularly those focusing on adolescent health, across diverse settings and contexts, and adolescents interested in contributing to health research.