This chapter covers the nomenclature and conceptual issues related to a radiologic and pathologic diagnosis of usual interstitial pneumonia (UIP) and the increasing acceptance of the idea that UIP is a radiologic/pathologic pattern that can be seen patients with idiopathic pulmonary fibrosis, fibrotic hypersensitivity, or some forms of connective tissue disease-associated ILD, as well as a drug reaction. Genetic abnormalities in UIP and attempts to use a genomic classifier for diagnosis are reviewed. Differences in pathologic findings in VATS and transbronchial cryobiopsies are discussed. Complications, including acute exacerbations, treatment, and prognosis are examined.