This guidebook is the most complete and up-to-date resource for identifying the subtidal marine invertebrates found along the west coast of North America, from the Gulf of Alaska to the Mexican border. It contains detailed information on 645 of the most commonly-encountered species, with notes on several hundred more.  An overview of each phylum is provided, followed by species descriptions that include a color photograph, important characteristics for field identification, details on how to distinguish it from similar forms, maximum size, known geographic range and habitat, and interesting facts related to the species. It is an essential reference for divers, tidepoolers, students, and photographers wishing to identify their finds, or anyone interested in the life of our oceans.