Reliance on close social partners for support and emotional security appears to be a fundamental component of human nature that emerges at the very beginning of life and plays a critical role in health and well-being over the lifespan. The present chapter provides an overview of contemporary social-psychological research on the basic functions and processes of close relationships. We place predominant emphasis on romantic ties, given that such relationships are usually adults' most intimate and important interpersonal relationships, and also given the extensive evidence that romantic relationships have particularly powerful and lasting effects on physical and mental well-being. We begin by briefly discussing various methods used in the close relationships field, after which we provide a detailed review of the major behavioral and cognitive processes that govern the formation and functioning of intimate relationships over the lifespan and the theoretical perspectives that have been used to explain these dynamics.