Introduction to Fuzzy Sets and Fuzzy Logic -- Algebras with Fuzzy Equalities -- Fuzzy Equational Logic -- Fuzzy Horn Logic.The book deals with similarity relations defined on a set with functions. The functions are required to map similar elements to similar ones. The book presents basic mathematical properties of structures consisting of similarity-preserving functions and logics for reasoning about similarities. The presented text is self-contained. The notions and results are demonstrated through examples which are graphically illustrated. The book is useful for researchers, but it can also be used as a graduate text.