This book delves into the dynamic and increasingly complex world of New Psychoactive Substances (NPS). Between 2009 and 2023, more than 1,250 distinct NPS were identified worldwide, with a record number reported in 2023 alone. This surge reflects a growing diversity, particularly in the categories of synthetic opioids (notably nitazenes), cannabinoids, and stimulants. The emergence of these substances presents significant challenges for public health, law enforcement, and regulatory frameworks, largely due to their novel chemical structures, limited detectability, and serious health risks, including addiction, mental illness, and overdose. Drawing on the most recent scientific and clinical evidence, this book offers a comprehensive examination of the pharmacological, medical, social, and legal aspects of NPS, with a focus on practical strategies for prevention, diagnosis, and treatment in the field of addiction medicine.