This volume represents the fourth in a series published following workshops of the international Hydrozoan Society. The book is an outcome of the IV Workshop, held at the Bodega Marine Laboratory in Bodega Bay, California, from September 19 to October 3, 1998. Through a series of selected papers, this volume covers a wide range of topics comprising traditional systematics, zoogeography, life cycles, ecology, behaviour, and evolution as well as molecular and biochemical approaches. This collection of papers, written by experts in their fields, makes this book a valuable addition to the Cnidaria literature. At the same time, the papers provide a good sample of the different research fields now alive in marine invertebrate biology.