The defining feature of neuroendocrine cells is secretion of amine or peptide hormones that regulate local physiological processes [1]. They act in concert with systemic neural and endocrine control systems and have features of both neurons and endocrine cells, hence their name. Whether scattered as individual cells within epithelia, as in the airways or gut, or aggregated into larger structures, such as the pancreatic islets, neuroendocrine cells are characterized by their morphological uniformity and high degree of organization. Their ultrastructural hallmark is the neurosecretory or dense core vesicles (DCVs) (Figure 3.1) that store and release their secretory products. Components of these and other secretory vesicles, together with a variety of membrane proteins and enzymes, endow neuroendocrine cells with a characteristic antigenic profile. Their integration with and contribution to the regulation of physiological processes are by a mode of local secretion known as paracrine [1–5].