Interest in identifying harmful dinoflagellate species has received worldwide recognition in recent years due to the increase in red tides, fish kills, and shellfish poisoning events reported of coastal marine ecosystems (Hallegraeff 1991). The publication, Identifying Harmful Marine Dinoflagellates, is an effort by the authors to present a fully illustrated identification guide for harmful dinoflagellate taxa. The user will recognize general information on dinoflagellate morphology and other criteria used in species identification. Each taxon is presented with a species overview, and a taxonomic description of cell and thecal plate morphology, reproduction, life cycle, ecology, toxicity, species comparison, habitat and locality, and etymology. This is supplemented with a number of high-resolution light and scanning electron photomicrographs and line drawings. Taxonomic treatment of harmful dinoflagellate taxa includes nomenclatural types, type locality, and common synonyms. The nomenclatural name of a species is taken from the original publication of the taxa, with the exception of those where the type species is not known. Species names used in this publication are valid as of those published by 2000. An extensive glossary of terms and relevant literature citations are also provided. This guide will be useful to teachers, researchers and students, as well as professionals involved in environmental water quality assessment and management, fisheries and aquaculture, and public health.