The tree-in-bud pattern is commonly seen at thin-section computed tomography (CT) of the lungs. It consists of small centrilobular nodules of soft-tissue attenuation connected to multiple branching linear structures of similar caliber that originate from a single stalk. Originally reported in cases of endobronchial spread of Mycobacterium tuberculosis, this pattern is now recognized as a CT manifestation of many diverse entities. These entities include peripheral airway diseases such as infection (bacterial, fungal, viral, or parasitic), congenital disorders, idiopathic disorders (obliterative bronchiolitis, panbronchiolitis), aspiration or inhalation of foreign substances, immunologic disorders, and connective tissue disorders and peripheral pulmonary vascular diseases such as neoplastic pulmonary emboli. Knowledge of the many causes of this pattern can be useful in preventing diagnostic errors. In addition, although the causes of this pattern are frequently indistinguishable at radiologic evaluation, the presence of additional radiologic findings, along with the history and clinical presentation, can often be useful in suggesting the appropriate diagnosis.