Algae are classified in an heterogeneous and very artificial group of living beings, with a very variable aspect. For the first time, this guide, full-colour illustrated by Calros Silvar, includes the description of the most important 90 seaweed of our coasts, the environment where they live and the relation that they establish with the other communities that make up the coastal ecosystem. It also includes some ideas about their culture and use, with identification keys at the level of genus. CD-Rom attached.