Social influence has been summarised as the change in one’s beliefs, behaviour, or attitudes due to external pressure that may be real or imagined (Cialdini, 2001). In this chapter, we focus on the question of how (sports-relevant) behaviour and athletic performances are influenced by others, especially active and passive (sports) spectators. You have probably already experienced giving a presentation in front of a group of people. Were you influenced by the presence of your audience? Was your performance better, worse, or unaffected compared to the rehearsal session, when you practised alone? Is your performance influenced differently when the audience listens attentively as opposed to when they are noisily not paying attention? How does this presence of others impact performances and behaviours in the context of sports? In sports, social influence has already been investigated extensively (cf. Epting, Riggs, Knowles, & Hanky, 2011; cf. Strauss, 2002b). A particular interest within social influence research is the home advantage in team sports (Allen & Jones, 2014; Carron, Loughhead, & Bray, 2005). Research in this field is concerned with understanding whether the performance of the home team is better due to more of their fans being in the stadium.