Opioids are powerful and widely used analgesics in modern medicine, playing a critical role in the treatment of both acute and chronic pain. This chapter provides an overview of opioid pharmacology, historical context, mechanisms of action, and clinical application. The chapter covers early natural derivatives like morphine and codeine to synthetic agents such as fentanyl and their use across diverse clinical settings. Opioids exert their effects through the mu (μ), kappa (κ), and delta (δ) opioid receptors, influencing both ascending and descending pain pathways. The chapter differentiates between short-acting and long-acting opioids, discussing their pharmacokinetic profiles, clinical indications, and conversion to morphine milligram equivalents (MME) for safe prescribing. This chapter also discusses the risks of tolerance, dependence, respiratory depression, and overdose, along with contraindications and patient-specific considerations. Clinical indications across acute, chronic, and palliative care are reviewed, as well as the unique role of partial agonists like buprenorphine. Finally, safe prescribing strategies to mitigate harm are discussed to ensure opioids are used safely and effectively as part of a multimodal pain management strategy.