22.1 Introduction
Aggregation of preferences, criteria or similarities happens at various stages in recommender systems. Typically such aggregation is done by using either the arithmetic mean or maximum/minimum functions. Many other aggregation functions
which would deliver flexibility and adaptability towards more relevant recommendations are often overlooked. In this chapter we will review the basics of aggregation functions and their properties, and present the most important families, including generalized means, Choquet and Sugeno integrals, ordered weighted averaging,
triangular norms and conorms, as well as bipolar aggregation functions. Such functions can model various interactions between the inputs, conjunctive, disjunctive
and mixed behavior. Following, we present different methods of construction of aggregation functions, based either on analytical formulas, algorithms, or empirical
data. We discuss how parameters of aggregation functions can be fitted to observed