Many family relationships are initiated and maintained through the exchange of affectionate behaviors such as hugging, kissing, hand holding, or saying "I love you". Why humans engage in affectionate behavior, and why it is associated with these benefits, are among the questions addressed by affection exchange theory (AET). AET begins with the proposition that the need and capacity for affection are inborn. This chapter describes the purpose and assumptions of AET and delineates its basic principles. It will also identify how AET conceptually defines communication, and reviews some of the research that has used AET to increase understanding of family relationships. it addresses the theory's strengths and limitations, and offer suggestions for future research and applications. AET conceptually defines only affectionate communication, rather than communication in general, although some broader concepts about communication can be derived from its approach.