In Encyclopedia of Artificial Intelligence. Information Science Reference, 2007. Distributed Constraint Reasoning Marius C. Silaghi, msilaghi@fit.edu Makoto Yokoo, yokoo@is.kyushu-u.ac.jp INTRODUCTION Distributed constraint reasoning is concerned with modeling and solving naturally distributed problems. It has application to the coordination and negotiation between semi-cooperative agents, namely agents that want to achieve a common goal but would not give up private information over secret constraints. When compared to centralized constraint satisfaction (CSP) and constraint optimization (COP), here one of the most expensive operations consists in communication and major problems arise from coherence and privacy. We review approaches based on asynchronous backtracking and depth-first search spanning trees. Distributed constraint reasoning started as an outgrowth of research in constraints and multi-agent systems. Take the sensors network problem in Figure 1, defined by a set of geographically distributed sensors that have to track a set of mobile nodes. Each sensor can watch only a subset of its neighborhood at a given time. Three sensors need to simultaneously focus on the same mobile node in order to locate it. Approaches modeling and solving this problem with distributed constraint reasoning are described in (Bejar, Domshlak, Fernandez, Gomes, Krishnamachari, Selman, &Valls, 2005). Figure 1: Sensor Network.