In recent years, physical activity has been widely accepted as a cornerstone of health, well-being, and quality of life. However, traditional exercise paradigms, which are often limited to indoor spaces or highly structured environments, have shown limitations in meeting the diverse physical, psychological, and social needs of different population groups. In response to these limitations, green exercise—defined as physical activity performed in natural settings—has emerged as a powerful and holistic approach that integrates movement with nature's healing properties. This book, Green Exercise: Adapted Physical Activity in Natural Environments, aims to provide a comprehensive scientific and practical framework for understanding how nature-based physical activity can be adapted for individuals with disabilities, older adults, and other special populations. Green exercise offers measurable benefits beyond physical health, including cognitive performance, emotional regulation, stress reduction, and overall well-being. These effects are increasingly supported by evidence from the fields of exercise science, environmental psychology, neuroscience, and public health. The main focus of this book is adapted physical activity, emphasizing inclusivity, accessibility, and individualization. Natural environments such as forests, parks, coastal areas, and green urban spaces offer flexible and scalable settings where physical activity can be modified according to functional capacity, health status, and personal needs. In this context, practices such as forest bathing (Shinrin-yoku) are examined not only as cultural or recreational activities but also as scientifically grounded interventions with psychophysiological significance. In addition to theoretical foundations, this book brings together hypothesis-driven perspectives, methodological considerations, and applied models to guide researchers, practitioners, and policymakers. Combining theory and practice, the book aims to contribute to the development of evidence-based interventions and inspire future research in the rapidly expanding field of nature-based exercise. Ultimately, this work advocates for a paradigm shift: a transition from viewing exercise as a purely mechanical action to understanding movement as an experience deeply connected to the natural world. We hope this book will serve as a reference for academics, health professionals, educators, and students aiming to design more inclusive, effective, and sustainable nature-based physical activity programs.