Peptide receptors, in general, are members of the large family of G-protein coupled receptors (GPCRs). Their endogenous ligands are neuropeptides and proteins of various kinds, such as chemokine (subfamily A1), vasopressin (subfamily A6), neurokinin (subfamily A9), and thrombin (subfamily A15). The diversity of endogenous ligands implies the wide range of biological processes they are involved in: include learning, memory, response to stress, pain, addiction, feeding behavior, sexual behavior, reproduction, the immune response, thermal control, kidney function, cardiovascular function (including blood pressure and heart rate) and many others. The discovery that peptide receptors (somatostatin (subfamily A4) in particular) are over-expressed in most human neuroendocrine tumors has focused the recent interest in peptide receptors as potential targets for the treatment of cancers.