Opioid and somatostatin systems are two main inhibitory systems in mammals implicated in a variety of processes from hormone secretion to the modulation of cell proliferation. Opioids and opioid receptors were found in a great diversity of primary human tumors and in various cancer cell lines. A new area of interest that has developed since the early 1980s has been concerned with the role of endogenous opioid systems in the growth of normal and abnormal tissues and cells. This review presents recent developments on the identification of opioid ligands and receptors in different types of human neoplasia. It also deals with the mechanisms of opioid peptide action in carcinoma and the involvement of opioids in the regulation of tumor growth.