This study explored the behavioral manifestations of sustained attention in 2-year-old children and the relation between sustained attention and its neurophysiological correlates. 51 children were administered a task of vigilance (the Early Childhood Vigilance Task; ECVT), along with behavioral measures of sustained attention that are routinely used as measures of attention in this age-group. A subset of these children, n = 14, was also evaluated in an event-related potential (ERP) paradigm to identify patterns of neurophysiological activation that might be associated with sustained attention. The results supported the conceptualization of vigilance as a component of sustained attention that is related to, but distinct from, attention toward objects or activities with which one is actively engaged. Results also provided support for a specific association between vigilance and right frontal brain activity. Whereas ECVT performance was associated with ERP activation in the right frontal area, performance on more traditional behavioral measures of sustained attention was not.