Hypersensitivity pneumonitis (HP) is a lung limited disease caused by an inhaled antigen. Major causes of HP are bird, mold, or farming exposures and contaminated humidifiers/air coolers, but there are many other causes that occur in small populations with specific exposures. The classification of HP has changed markedly, and the old categories of acute, subacute, and chronic HP are no longer used; rather, all HP cases are classified as either nonfibrotic or fibrotic. Nonfibrotic HP is typically characterized by peribronchiolar (centrilobular) interstitial inflammation and giant cells/granulomas, whereas fibrotic HP can look like fibrotic NSIP, UIP, centrilobular fibrosis, or a mixture of a UIP and centrilobular pattern. Some cases of fibrotic HP have giant cells/granulomas, but many do not, and some cases of fibrotic HP are morphologically inseparable from UIP/IPF. The prognosis of nonfibrotic HP is believed to be good with antigen removal and immunosuppressive therapy, but the prognosis of fibrotic HP is guarded, and many cases fall into the general area of progressive pulmonary fibrosis.