Interstitial lung diseases (ILDs) are a heterogeneous group of disorders characterized by inflammation and/or fibrosis of the lung parenchyma. Idiopathic pulmonary fibrosis (IPF) is the most common and most severe ILD, characterized by usual interstitial pneumonia (UIP) pattern: patchy, peripheral, basal-predominant fibrosis with honeycombing and fibroblastic foci. The pathogenesis involves repetitive alveolar epithelial cell injury triggering aberrant wound healing — activated myofibroblasts deposit excessive extracellular matrix (collagen) causing progressive, irreversible scarring. Other ILDs include connective tissue disease-associated (CTD-ILD — especially SSc, RA, myositis), hypersensitivity pneumonitis (HP — from organic antigen exposure: bird proteins, mold, chemicals), sarcoidosis, drug-induced ILD, and pneumoconioses (asbestosis, silicosis). HRCT pattern recognition (UIP vs NSIP vs organizing pneumonia) guides diagnosis and management. PFTs show restrictive pattern with reduced DLCO, which is often the earliest abnormality.