The diagnosis of UPAA is made via imaging. An abnormal finding on chest radiograph prompts investigations regarding UPAA. Chest radiographs can show absent hilar shadow, ipsilateral hypoplastic lung, ipsilateral mediastinal shift, ipsilateral decreased pulmonary markings, and hyperinflation of the contralateral lung. An echocardiogram is helpful in excluding pulmonary hypertension, valvular anomalies, septal defects, or congenital heart disease. Scintigraphy can evaluate the anatomy of the affected side and the ventilation/perfusion status of the lungs. The constellation of findings in a ventilation/perfusion scan in UPAA consists of reduced perfusion to the whole affected lung coupled with diminished ventilation. CT scans or MRIs of the chest with contrast offer the definitive diagnosis of UPAA because they provide detailed anatomy, can evaluate congenital heart defects, and can also diagnose underlying pulmonary infections and bronchiectasis. Pulmonary angiography remains the gold standard for diagnosis of UPAA, but its use has decreased with the more widespread use of echocardiography, chest CT scans, and MRIs. Pulmonary angiography is reserved for therapeutic purposes such as embolization for hemoptysis or if revascularization is proposed by visualization of hilar arteries.