This book aims to be a comprehensive and accurate survey of state-of-art research on intuitionistic fuzzy sets theory and could be considered a continuation and extension of the authorś previous book. The research activity of the author within the area of intuitionistic fuzzy sets has been expanding into many directions. The results of the authorś most recent work covering the past 12 years as well as the newest general ideas and open problems in this field have been therefore collected in this new book.