The theory of fuzzy sets has developed in numerous ways and across many fields since its conception. Artificial intelligence, computer science, control engineering, decision theory, expert systems, logic, management science, operations research, robotics, and other fields all use fuzzy technology in some way. Numerous theoretical advancements have been made. Fuzzy Set Theory - and its Applications, Fourth Edition's main objective is to serve as both an introduction to fuzzy set theory and a textbook for courses in the subject. Numerous helpful references have been incorporated to help the interested reader gain a better knowledge in order to strike a balance between the character of a textbook and the dynamic nature of this study. The fourth edition of Fuzzy Set Theory and its Applications revises the research agenda with chapters on possibility theory, fuzzy logic and approximate reasoning, expert systems, fuzzy control, fuzzy data analysis, decision making and fuzzy set models in operations research. Chapters have been updated and extended exercises are included.