Determining the etiology of pediatric pulmonary hypertension is essential to appropriate management. Assessment of the patient requires complete history and physical examination as well as the use of investigative modalities including echocardiography, cardiac catheterization, cardiac magnetic resonance imaging (MRI), chest computed tomography, and lung biopsy. This review summarizes recommendations for diagnostic work-up and includes a clinical algorithm for evaluation of the patient with acute pulmonary hypertension in the pediatric intensive care unit.