In this chapter, we discuss attachment theory and our programs of research on how individuals with different adult attachment orientations think, feel, and especially behave when they and their romantic partners encounter different types of stressful situations. In Section 2, we review some basic principles of attachment theory, discuss what adult attachment orientations are, and summarize what they correlate with in the context of relationships. We also review how the different adult attachment orientations are associated with how individuals regulate negative emotions in threatening situations. In Section 3, we discuss diathesis-stress thinking in attachment theory, and we introduce the general diathesis-stress process model that has guided most of our research on adult attachment, stress, and relationships during the past 20. years. In Section 4, we review the various programs of attachment research we have conducted, which have tested how adults who have different attachment orientations cognitively, emotionally, and behaviorally react to different types of threat/stress vis-à-vis their romantic partners and relationships. We conclude the chapter by summarizing our diathesis-stress findings and by discussing promising directions for future research.