Periwinkles are snails belonging to the genus Littorina, common on rocky sea shores of Western Europe, Scandinavia, North America, Siberia and Japan. Littorina is one of the most thoroughly studied of all marine gastropod genera. Reid lists almost 1500 references at the end of his book, of which 1300 are concerning Littorina species. This work is unique among Littorina literature, and even among marine molluscs in general, since it compiles all known information about all known fossil and living species of a single genus, including taxonomy, shell variation, morphology, reproductive anatomy, habitat, distribution, biology, ecology, allozyme frequencies as well as DNA sequences. Information is based on both original research and the above mentioned bibliography. The book is clearly structured. The introduction provides a historic perspective of research on Littorina. The chapter ‘Material and Methods’ is exceptionally thorough as all morphological characters from both the soft parts and the shell (teleoconch) that are most useful for taxonomic discrimination and phylogenetic analysis of Littorina species are described and evaluated at length. Since the comparatively stable anatomical features are most important to identify unequivocally the various (sibling) species, and the extreme variability in shell features provide excellent possibilities for micro-evolutionary research; the comments on the causes of variation are also of great importance. The main part of Reid’s magnificent work is formed by the systematic descriptions. All available knowledge is presented consistently. A typical treatment of a species (here L. littorea) consists of synonymy, taxonomic history, a diagnosis, the anatomical description (head-foot, penis, sperm, oviduct, spawn and development, radula and foregut), shell variation (geographical variation, ecotypic variation, sculptural variation, sexual dimorphism, recent monstrosities, Norwich Crag monstrosities, colour variation), distribution (habitat, range, spread in North America, fossil record), discussion (intraspecific variation, spread in North America, phylogenetic relationships, similar species). This occupies 25 pages. Littorina saxatilis needs 62 pages, of which 9 are used for synonymy only. Whenever confusion between species is most likely, the description of the most discriminative characters is tabulated.