The primary role of the thymus lies in T-cell differentiation and self-education leading to the establishment of appropriate host immune defenses. However, the view of the thymus as a self-contained organ is no longer valid. It is now clear that intricate interactions of both a stimulatory and inhibitory nature exist between the neuroendocrine and immune system. A broad array of neuroendocrine circuits are networked with the thymus and neuroendocrine-thymic interactions are bidirectional. These interactions are thought to play an important immunomodulatory role during an active immune response, during T-cell ontogeny and in the aging process of the whole organism. The chemical messengers that transmit communicating signals in this network are secreted neuropeptides and their specific receptors. The objective of this review is to provide a comprehensive overview of the morphological substrates of these neuropeptides in the thymus.