Pheochromocytomas and paragangliomas are rare but clinically important chromaffin cell tumors that typically arise from the adrenal gland or from extra-adrenal paraganglia, respectively and constitute a surgically correctable cause of chronic hypertension. The clinical features and consequences of pheochromocytoma/paraganglioma result from the release of catecholamines (e.g., norepinephrine and epinephrine) by the tumor. If a pheochromocytoma/paraganglioma is undetected, stimuli that normally would not pose a hazard, such as surgery, childbirth, or general anesthesia, can evoke catecholamine secretion by the tumor, with clinically significant and even catastrophic outcomes. The diagnosis of pheochromocytoma/paraganglioma and its localization can be challenging, because measurements of plasma levels or urinary excretion of catecholamines and their metabolites as well as radio-iodinated metaiodobenzylguanidine (MIBG) scanning can yield false-positive/negative results in patients harboring the tumor. Computed tomography (CT) and magnetic resonance imaging (MRI) lack sufficient specificity. The molecular mechanisms by which genotypic changes predispose to the development of pheochromocytoma/paraganglioma remain unknown, even in patients with identified mutations. Moreover, pheochromocytomas/paragangliomas in patients with hereditary predispositions differ in terms of their growth, malignant potential, catecholamine phenotype, responses to standard screening tests, such as the clonidine suppression test, various imaging modalities, and different therapeutic options. This protocol focuses on developmental, molecular, genetic, epigenetic, proteomic, metabolomics, and other types of studies to elucidate the bases for predisposition to develop pheochromocytomas/paragangliomas and for expression of different neurochemical phenotypes and malignant potentials including therapeutic responses. Furthermore, this protocol will also use new imaging approaches, for example [(18)F]-6F-dopamine ([(18)F]-6F-DA), [(18)F]-L-3,4-dihydroxyphenylalanine ([(18F)]-DOPA), and [(68)Ga]-DOTA-Tyr-octreotate ([(68)Ga]-DOTATATE) positron emission tomography (PET)/CT, as well as PET/MRI scanning, and dynamic contrast-enhanced MRI. Additionally, new biochemical diagnostic criteria exist for the measurement of plasma metanephrines and methoxytyramine for the clinical diagnosis and localization of these tumors. This protocol will also evaluate the benefits of histone deacetylase inhibitors (e.g. romidepsin) pretreatment for uptake enhancement of [(123/131)I]-MIBG in pheochromocytoma/paraganglioma tumors.