Opioid medications are an indispensable part of contemporary pain management. These drugs are extensively used to treat acute and chronic malignant and noncancer pain in inpatient and outpatient settings on a daily basis. Given their broad utilization in clinical practice, a thorough familiarity with opioid analgesics is of utmost importance. This chapter will provide a general overview of opioid medications, their classification systems, mechanisms of action, and common examples of opioids encountered in everyday practice. Opioids are grouped, based upon their action at opioid receptors, into three separate categories following as agonists, mixed agonists-antagonists, and antagonists.