This book constitutes the refereed proceedings of the 6th International Conference on Integration of AI and OR Techniques in Constraint Programming for Combinatorial Optimization Problems, CPAIOR 2009, held in Pittsburgh, PA, USA, in May 2009.

Invited Talks.- Machine Learning Framework for Classification in Medicine and Biology.- G12 - Towards the Separation of Problem Modelling and Problem Solving.- Regular Papers.- Six Ways of Integrating Symmetries within Non-overlapping Constraints.- Throughput Constraint for Synchronous Data Flow Graphs.- A Shortest Path-Based Approach to the Multileaf Collimator Sequencing Problem.- Backdoors to Combinatorial Optimization: Feasibility and Optimality.- Solution Enumeration for Projected Boolean Search Problems.- k-Clustering Minimum Biclique Completion via a Hybrid CP and SDP Approach.- Optimal Interdiction of Unreactive Markovian Evaders.- Using Model Counting to Find Optimal Distinguishing Tests.- Reformulating Global Grammar Constraints.- IBM ILOG CP Optimizer for Detailed Scheduling Illustrated on Three Problems.- Open Constraints in a Boundable World.- Sequencing and Counting with the multicost-regular Constraint.- Bandwidth-Limited Optimal Deployment of Eventually-Serializable Data Services.- Tightening the Linear Relaxation of a Mixed Integer Nonlinear Program Using Constraint Programming.- The Polytope of Context-Free Grammar Constraints.- Determining the Number of Games Needed to Guarantee an NHL Playoff Spot.- Scalable Load Balancing in Nurse to Patient Assignment Problems.- Learning How to Propagate Using Random Probing.- DFS* and the Traveling Tournament Problem.- Max Energy Filtering Algorithm for Discrete Cumulative Resources.- Extended Abstracts.- Hybrid Branching.- Constraint Programming and Mixed Integer Linear Programming for Rescheduling Trains under Disrupted Operations.- Constraint Models for Sequential Planning.- A Fast Algorithm to Solve the Frequency Assignment Problem.- A Hybrid LS/CP Approach to Solve the Weekly Log-Truck Scheduling Problem.- Modelling Search Strategies in Rules2CP.- CP-INSIDE: Embedding Constraint-Based Decision Engines in Business Applications.- An Integrated Genetic Algorithm and Integer Programming Approach to the Network Design Problem with Relays.- A Benders'Approach to a Transportation Network Design Problem.- Progress on the Progressive Party Problem.