Opioids play a crucial role in managing acute pain due to their high efficacy and impressive safety profile, including the advantage of reversibility. Derived from the extraction of morphine from the poppy plant, opioids have been prized for their analgesic effects for centuries, with morphine and its derivatives continuing to be essential in pain management. These drugs attach to opioid receptors located in peripheral and central nervous systems, inhibiting the release of excitatory neurotransmitters and hyperpolarizing the neuronal membrane. This process diminishes pain from nociceptive stimuli while preserving proprioception. Opioid receptors (OR) are essential for pain modulation, addiction, and various physiological functions, binding with naturally produced or externally introduced opioids. Opioid receptors, belonging to the G protein-coupled receptor (GPCR) family, are expressed extensively all through the central and peripheral nervous systems, as well as in various other tissues. Structurally, these receptors exhibit distinct features, including glycosylation and phosphorylation sites, within their seven-transmembrane domains. Naturally occurring endogenous opioid peptides such as enkephalins, endorphins, dynorphins, and endomorphins modulate OR activity. Moreover, exogenous opioid peptides, known as exorphins, derived from external sources such as food proteins, exert morphine-like effects when metabolized in the body. In this chapter, we focus on the intricate mechanisms of OR endocytosis and signaling, highlighting their critical roles in pain modulation, addiction, and broader physiological functions. Research into OR signaling holds promise for the development of more potent opioid therapeutics in terms of safety and efficacy, offering insights into novel targets and strategies for pain management and addiction treatment.