Book excerpt: Nudibranchs, the ‘butterflies of the sea’, belong to a group that includes bubble shells, sea hares, side-gilled slugs, sap-sucking slugs and sea butterflies (pteropods). This group includes some of the most beautiful, colourful and delicate of all marine creatures. More than 400 species of nudibranchs occur in south-eastern Australia. This guide introduces marine naturalists, divers, biologists and others to the nudibranchs and related molluscs commonly encountered in the Bass Strait region—their identification, biology, and associations with other plants and animals. An introductory pictorial key is included, along with nearly 250 species descriptions accompanied by colour photographs and illustrations to aid recognition. Further references and a glossary are also included.