The Fourth Edition of this best-sellng text provides an introduction to theory and applications of fuzzy logic. This text will appeal to instructors in mathematics, computer science, and engineering. It has been updated and remains a timely introduction to the mathematics needed for designing intelligent systems. The new edition includes fuzzy sets of type-2, additional materials on cupolas, more coverage of algorithms, and historical material revealing the development of the topic. The text remains a timely choice for courses offered in several discplines and a reference for instructors and students alike.