The conantokins belong to a distinctive family of Conus peptides which are known to have sleepinducing activity in young mice, and are found to be inhibitors of N-methyl-D-aspartate (NMDA) receptors. This paper presents an overview of the various conantokins characterized so far, with focus on the biochemistry, solution structures in the absence or presence of divalent cations, function as selective NMDA receptor antagonists, and potential as therapeutic agents for some neurological conditions associated with NMDA receptor dysfunction.