Serotonin receptor research has demonstrated that serotonin (5-HT) interacts with different (sub) types of receptors. Many compounds display a high affinity for 5-HT receptors, but only a few are selective (ratio > 100) for one of the 5-HT receptor subtypes. At present potent and selective ligands have been developed for the 5-HT_1A, 5-HT_1B, 5-HT₂, 5-HT₃ and 5-HT₄ receptors. For the 5-HT_1C, 5-HT_1D, 5-HT_1E and the recently discovered 5-HT_1F, 5-HT₅ and 5-HT₆ receptor subtypes only potent non-selective compounds are available. In this paper was present the affinities (obtained by radioligand binding studies) of the most selective ligands known today for each of the 5-HT subtypes and discuss the structure-activity relationships of some interesting series. The potential use of several of these selective ligands as pharmacological tools and therapeutics will be briefly reviewed.