Attachment research has traditionally focused on traits, enhancing our understanding of attachment-related individual differences. However, to chart the dynamic properties of the attachment system, more research is needed on the within-person fluctuation of attachment states. In this ecological momentary assessment (EMA) study, we examined (a) the associations between the baseline, variability, and inertia of each state attachment dimension (security, avoidance, and anxiety) and (b) how trait attachment (anxiety and avoidance) predicts these dynamic features.