Any informal communication about a product or brand by consumers on the internet can be defined as electronic word of mouth (eWOM). Research has shown that eWOM is perceived to be more credible by consumers and can increase sales. eWOM is a part of social interactions among individuals and it occurs among individuals with varying degrees of interpersonal closeness, the perceived level of psychological bond between two individuals. This PhD thesis aims to investigate how eWOM from senders with different degrees of interpersonal closeness influences consumers. Since the foundation of interpersonal closeness is interdependence among individuals, the three papers are based on theories related to cognitive, emotional, and behavioral interdependence. The first paper focuses on emotional interdependence among the sender and the audience and investigates how the emotional valence of eWOM influences the audience and subsequently their purchase intention. The second paper focuses on behavioral interdependence, the act of self-disclosure and reciprocity, among the sender and the audience in social media-based brand community contexts. Finally, the third paper investigates how eWOM triggers social comparison and how cognitive interdependence influences the social comparison process and subsequently, purchase intention.