This book constitutes the refereed proceedings of the 9th International Conference on Spatial Information Theory, COSIT 2009 held in Aber Wrac'h, France in September 2009. The 30 revised full papers were carefully reviewed from 70 submissions. They are organized in topical sections on cognitive processing and models for spatial cognition, semantic modeling, spatial reasoning, spatial cognition, spatial knowledge, scene and visibility modeling, spatial modeling, events and processes, and route planning.