This study examined the developmental significance of adult attachment security--as measured by the Adult Attachment Interview--for romantic relationship functioning concurrently and approximately 1 year later in a sample of heterosexual dating couples between the ages of 18 and 25 (115 dyads at Time 1 [T1] and 57 dyads at T2, 74% White). The authors assessed romantic relationship functioning at T1 and T2 using observers' ratings of emotional tone during a laboratory conflict resolution task and via participants' self-reports about their relationships, yielding evidence that adult attachment security prospectively predicted the observed and perceived quality of adults' romantic relationships even after prior levels of interpersonal functioning were controlled. Measures of autonomic responding were also acquired during the interactions, replicating prior evidence that insecurity is concurrently associated with electrodermal reactivity in attachment-relevant contexts. (Contains 1 footnote and 2 tables.)