Since the 1960s, researchers have taken major strides toward understanding the role of self-disclosure in the initiation, development, maintenance, and ending of relationships. In the current chapter, we review theoretical and empirical milestones in our understanding of self-disclosure. We show that empirical research of self-disclosure shifted from a focus on the individual to a focus on the inherently interpersonal nature of disclosure processes. This shift marks the increasing awareness that self-disclosure happens between people. It elicits a cyclical process in relationships, which is specific to a particular relationship with a particular partner. Self-disclosure is a dynamic process that shapes, and is shaped by, relationships. It serves as a monitor for relationship quality and is moulded by the context and the medium in which it takes place. It is essential in interdependent relationships and key to unraveling how people discern the quality of their relationships. Despite the strides research has made in our understanding of self-disclosure, throughout the chapter, we identify unanswered questions which provide promising avenues for future research.