Reasoning Methods on Data with Incomplete Description (in Polish) Rafal Latkowski Warsaw Univerity, Institute of Computer Science, 2001 Master thesis under supervision of Professor Andrzej Skowron 89 pages, 65 references, 4 tables, 7 figures Abstract This work contains an overview of methods that allow reasoning on data with incomplete object description. Presented here methods consider both, imputing the missing values, as well as directly reasoning on data with incomplete object description. The intention of the author was to provide as complete as possible roadmap of solutions applied in intelligent data analysis and knowledge discovery together with pointing out their origins. First chapter introduces the problem of intelligent data analysis using data with incomplete object description. Second chapter provides basics of Rough-Set theory. Third chapter presents extension of the Rough-Set theory to the case of missing attribute values. Fourth chapter presents the inductive learning methods that deal with problem of missing attribute values beyond the Rough-Set framework. Fifth chapter describes lazy learning methods dealing with problem of incomplete object description. Sixth chapter presents imputation methods. Seventh chapter presents new approach, the Decomposition Method that provides a possibility to adapt any learning method to the case of missing attribute values. Presented empirical evaluation shows good efficiency of this method. Final chapter contains conclusions.