Interdependence theory is a powerful and applicable theory that has shaped the study of interpersonal relationships for decades, providing foundational constructs and elucidating key assumptions within the burgeoning field of relationship science. Research guided by interdependence theory sheds light on the diverse phenomena within ongoing relationships, including the emergence of co-operation, trust, dependence, power, and relationship maintenance. At its core, interdependence theory pinpoints key elements of daily interactions that predict specific experiences and outcomes that people have in different situations. This handbook goes further to explain how interdependence theory continues to be used fruitfully in research, driving our current understanding of relational processes. We invite you to enter the world of interdependence and discover what top scholars across disciplines are discussing in their efforts to fully understand close, intimate relationships.