One basic tenet of attachment theory is that interactions with loving, caring, and supportive relationship partners and the resulting sense of attachment security (confidence that one is socially valued and that others will be available and helpful when needed) are building blocks of mental health and social adjustment. Moreover, attachment theory provides a valuable conceptual framework for the study of optimal relationship development. In this chapter, we first present a brief overview of attachment theory and our model of attachment processes in adulthood, an extension of Bowlby’s theory that is now supported by 25 years of research by personality and social psychologists. Next, we focus on the anchoring of attachment security in expectations concerning relationship partners’ sensitivity and responsiveness (expectations organized within a secure-base script), and review research findings showing that the sense of security is associated with positive appraisals of self and others and with social adjustment. We then review studies of dating and marital relationships showing that dispositional and experimentally induced feelings of security can transform otherwise destructive reactions to relationship partners’ undesirable, hurtful behavior, improve a person’s handling of interpersonal conflict, foster empathy and support for a partner in need, and promote positive relational emotions.