In social interactions, humans use nonverbal forms of behavior, performed by different body parts (e.g., head, eyes, hands), in addition to verbal behavior. Many of these forms of nonverbal behavior are multifunctional and their meaning is not unequivocal. This may raise communication challenges not only in human–human interaction but also in human–machine interaction, especially when the interaction partners are not able to interpret the nonverbal behavior and their functions correctly. This chapter focuses on different forms and functions of human nonverbal behavior by reviewing research both on human–human and human–machine interaction. The purpose of the chapter is to explain the importance and relevance of human nonverbal behavior for the development of sXAI and to suggest how to leverage characteristics of human nonverbal behavior to possible scenarios of human–machine interaction.