This book deals with a computer-supported medical diagnosis with particular focus on ovarian tumor diagnosis. The book presents theoretical foundations (both medical and mathematical) of the intelligent OvaExpert system which supports the decision-making process in tumor diagnosis. The main purpose for creating OvaExpert was to support a gynecologist in the process of predicting the malignancy of ovarian tumors by applying the existing diagnostic models and using modern methods of computational intelligence which allow for imprecision and imperfection of the medical data, both of which are common features of the everyday medical practice. The authors had good reasons to focus on this particular cancer. Ovarian cancer is difficult to diagnose and has high mortality, especially in the Central and Eastern Europe. The book presents novel methods based on interval-valued fuzzy sets and the theory of their cardinalities.