Difficulty managing the stress of conflict in close relationships can lead to mental and physical health problems, possibly through dysregulation of the hypothalamic-pituitary-adrenal (HPA) axis, the neuroendocrine stress response system. Temperament, an individual characteristic, and attachment, a dyadic characteristic, have both been implicated in emotion regulation processes and physiological reactivity, yet there is no clear consensus on how the two work together to influence the stress response, especially after childhood. The present study investigated the ways in which temperament and attachment together predict HPA response in emerging adult couples. Analyses using multilevel modeling (HLM) found that partners' dyadic fit on attachment avoidance impacted females' cortisol response patterns, and attachment avoidance further moderated the effect of males' emotionality on both their own and their partners' cortisol. Results are discussed in terms of emotional coregulation processes in romantic attachment.Although the experience and regulation of emotion has generally been considered a personal phenomenon, there is growing recognition of the importance of interpersonal relationships for the amplification and repair of emotional states (Diamond & Aspinwall, 2003). More attention has been focused on psychobiological regulation processes in infant-caregiver dyads (e.g., Schore, 1996), yet intimate partnerships throughout life provide a context for the regulation of emotional and physical well-being (Cacioppo, 1994;Hofer, 1984), making biobehavioral coregulation in adult romantic dyads a key target for further study. The way in which a person or couple responds to stressful situations has important implications for mental and physical health; an inability to cope with the stresses that inevitably arise within relationships sets the stage for distressed relationships, which in turn increase the risk for internalizing disorders (Coyne et al., 2002;Davila et al., 2003) and morbidity/mortality (e.g., Helgeson, 1991;Hibbard & Pope, 1993). Physiological stress response, as measured by the output of the hypothalamicpituitary-adrenal (HPA) axis, offers a promising route by which relationship stress might lead to such disorders, given that dysregulation of this system consistently characterizes depressive and/or anxiety disorders (e.g., Butler & Nemeroff, 1990;Young et al., 2004), as well as impaired cardiovascular and immune function (Glaser & Kiecolt-Glaser, 1994;Kuhn, 1989). To understand differences in couples' responses to conflict and the health implications of these responses, we propose a biopsychosocial model of emotion regulation, in which a combination of intra-and interpersonal characteristics shapes one's response to stressors across various psychobiological systems. In this paper, we seek to illuminate a central part of the pathway from interpersonal stress to health outcomes by investigating how components of romantic Correspondence concerning this article should be addressed to Heidemarie Laurent...