This book constitutes the proceedings of the 6th International Conference on Modeling Decisions for Artificial Intelligence, MDAI 2009, held on Awaji Island, Japan, in November/December 2009. The 28 papers presented in this book together with 5 invited talks were carefully reviewed and selected from 61 submissions. The topics covered are aggregation operators, fuzzy measures and game theory; decision making; clustering and similarity; computational intelligence and optimization; and machine learning.