This review gives a historical perspective, summarizing approximately 25 years of research on opioids. The "typical" opioid peptides produced in the brain, "atypical" opioids encrypted in milk protein or hemoglobin sequences, and extremely potent and selective opioids of amphibian origin are described. The main focus is on the structure-activity relationship studies of peptide ligands for three main opioid receptor types (micro, delta, kappa), their selectivities and pharmacological activities in vitro. Chemical modifications that led to obtaining potent and selective agonists and antagonists for these receptors are discussed.