Tissue damage is a state of injury or alteration affecting the cellular and structural integrity of biological tissues, resulting in impaired function. As a fundamental concept in biology and medicine, it involves the systematic investigation of the diverse etiologies (including mechanical, chemical, thermal, infectious, and immune insults), the complex molecular and cellular mechanisms driving cellular degeneration and extracellular matrix disruption, the subsequent pathological changes and functional deficits, and the inherent biological processes of inflammation, repair, and regeneration. Understanding tissue damage is critical for diagnosing and treating a wide range of diseases, managing trauma, and developing regenerative therapies.