This book constitutes the refereed proceedings of the International Conference on Spatial Information Theory, COSIT '99, held in Stade, Germany, in August 1999. The 30 revised full papers presented were carefully reviewed and selected from 70 submissions. The book is divided into topical sections on landmarks and navigation, route directions, abstraction and spatial hierarchies, spatial reasoning calculi, ontology of space, visual representation and reasoning, maps and routes, and granularity and qualitative abstraction.