This volume contains the papers presented at the Ninth International Conference on Principles of Knowledge Representation and Reasoning. Knowledge representation and reasoning (KR&R) lies at the crossroads between artificial intelligence research and modern information technology driven by the World Wide Web. Its ambition is to pave the road to the possibility of storing, retrieving and exploiting human knowledge by means of computers in a rational and efficient way. It relies on a blend of logics, graphical representations, and other formal languages for representing and reasoning about one’s and other agent’s beliefs about time, space, motion, as well as more specific domains of interest. KR&R is multidisciplinary and brings together philosophers, linguists, and computer scientists. KR&R provides crucial foundation for a wide range of applications with tremendous potential economic impact.