The California serogroup viruses are important human pathogens, originally discovered in the United States, but now recognized to occur and cause disease in many parts of the world. In addition to their significance to public health, study of their natural history has resulted in a better understanding of the way in which arboviruses are maintained in nature. Viruses of the California serogroup have also been carefully investigated on the molecular level, resulting in a better appreciation of how viruses are constructed, organized, and replicate. William C. Reeves, in whose honor this symposium is held, has contributed throughout his career to each of these fields of endeavor. His pioneering work on these viruses, beginning with the discovery of the prototype California encephalitis virus, has helped to establish the foundations upon which our current understanding of these viruses is built.