Chronic pain is a complex, multifactorial condition that persists beyond normal tissue healing time and significantly impairs physical, psychological, and social functioning. This chapter provides an overview of the definitions, classifications, pathophysiology, and impact of chronic pain. Chronic pain can be categorized into nociceptive, neuropathic, and nociplastic subtypes, although many patients present with mixed pain conditions. The underlying mechanisms involve both peripheral and central sensitization, neuroinflammation, dysregulation of neurotransmitter systems, and structural and functional changes in the nervous system. These biological changes are compounded by psychosocial factors such as pain catastrophizing, kinesiophobia, and coexisting mental health conditions including anxiety and depression. The chapter also discusses the substantial burden chronic pain places on individuals and healthcare systems, including reduced quality of life, impaired physical and social function, and increased economic costs. The chapter emphasizes the need for individualized, multidisciplinary care and greater attention to the psychological dimensions of chronic pain.