One of the primary functions of the attachment behavioral system is to regulate emotional experience under conditions of threat. Although research supports this association among infants and adults, few studies examine the relation between emotion and attachment in middle childhood. This study examined the concurrent associations among children's attachment organization and three indices of emotion reactivity/regulation: self- and parent-assessments of emotion, neuroendocrine reactivity, and fear-potentiated startle response. Ninety-seven 8- to 12-year-old children completed the Child Attachment Interview (CAI) and a fear-potentiated startle paradigm on separate occasions, with salivary cortisol assessed before and after each assessment. Greater attachment security was related to greater child-reported positive trait- and state-level emotion, lower pre-CAI cortisol levels, higher initial startle magnitude during threat, and a faster decrease in startle magnitude during threat. The findings provide initial support that attachment security is related to select measures of emotion, though different methods of assessment yielded discrepant findings. The findings are discussed in terms of their contribution to theory and research examining attachment and emotion. © 2010 American Psychological Association.