Opioids are responsible for the vast majority of preventable deaths related to drug use, and opioid crisis is a serious international health problem. Opioids are also the mainstay in pain treatment and are invaluable tools in palliative care. The purpose of this book is to present an overview of the complex field of opioid pharmacology through evidence-based chapters in clear language backed up with sound scientific evidence, providing researchers and health professionals with a firm understanding of their effects and mechanisms of action. The authors present an overview of the history of opioids from ancient civilizations to the current opioid crisis, covering state-of-the-art advances on opioid-induced signal transduction, opioids in pain management, and the neuroinflammatory effects of opioids. Also discussed are opioid use disorders and their treatment.