This book describes the current status of major parts of the theory of aggregation operators and examines potential areas for its applications. The authors have gathered self-contained papers and present essential material on selected topics in a comprehensive manner. In doing so they avoid tedious proofs and unnecessary details - this is not the case for several contributions containing new results and ideas which show the latest streaming of aggregation theory and potential application areas. The book is divided in four parts. The first part contains an overview of aggregation operators, including construction methods and different classes of aggregation operators. The second part is devoted to the theoretical aspects of aggregation and the third part looks at the field of applications. The final part offers an alternative look at the aggregation of verbal variables as fuzzy quantities.