Close relationships are an important and desired aspect of the human experience; however, as individuals pursue intimacy and connection with others, they will encounter a variety of questions about the nature, status, and future of their relationships. Consequently, uncertainty is an inevitable and unavoidable element of close relationships. It can arise in response to a variety of relational circumstances and can shape the ways that partners think, feel, and act toward one another. This book summarizes the expansive body of theoretical and empirical research regarding the nature of uncertainty, the conditions that promote uncertainty about relational involvement, and the emotional, cognitive, and communicative outcomes of uncertainty for individuals and their relationships. Based on the robust accumulation of data about uncertainty in close relationships, the book also offers recommendations for coping with ambiguous relational circumstances and proposes new directions for conceptualizing and studying uncertainty in close relationships.