The three-dimensional structures of peptide and protein toxins from the sea are of interest for a variety of reasons. They adopt many of the structural folds found in their terrestrial counterparts, as expected, but also have some that are unique. They are valuable structural probes of their cognate receptor binding sites, frequently on ion channel proteins, and play important roles in mapping these binding sites and in testing and refining models of receptor structures. Furthermore, the often exquisite selectivity of toxins for certain subtypes of a given receptor makes them attractive as potential therapeutic agents, in which case a knowledge of their structure becomes an essential part of subsequent drug design strategies. This review summarises current information on the structures of marine toxins, determined by either X-ray crystallography or, more often, nuclear magnetic resonance spectroscopy. Where the amino acid residues essential for activity have been identified, or where the toxin binding site on a receptor has been mapped, these results are also discussed.