Venomous marine cone snails harbour a variety of small disulfide-rich peptides called conotoxins, which target a broad range of ion channels, membrane receptors, and transporters. More than 700 species of Conus are thought to exist, each expressing a wide array of different peptides. Within this large repertoire of toxins, individual conotoxins are able to discriminate between different subtypes and isoforms of ion channels, making them valuable pharmacological probes or leads for drug design. This review gives a brief background to the discovery of conotoxins and describes their sequences, biological activities, and applications in drug design.