The fusion of identification guide and biological text, the hallmark of A Student's Guide to the Seashore, has been widely acclaimed since the publication of the first edition in 1989 and over the past 20 years we have been greatly encouraged by the support and encouragement received from users of the Guide. It was therefore a pleasure to accept the invitation from Cambridge University Press to prepare a third edition.

The publication of a third edition has given us the opportunity to review the list of species included in the light of our field experience and that of colleagues. Additional species have been added, some of which are introduced species in the sense that they have now become established in Britain and north-west Europe, bringing the total coverage to over 650 species. At the same time, the biological information on individual species has been updated on the basis of the latest research and expanded wherever possible and this, together with the extended bibliography of primary sources of data, will encourage wider lines of enquiry and help those readers who wish to investigate further this fascinating habitat. One of the most significant changes since the publication of the earlier editions has been in taxonomy following the increased use of molecular techniques and there is no doubt that important advances in our understanding of taxonomic relationships will continue to be made through the application of molecular phylogeny.