Detection of novelty is an important cognitive ability early in development, when infants must learn a great deal about their world. Work with adults has identified networks of brain areas involved in novelty detection; this study investigated electro-physiological correlates of detection of novelty and recognition of familiarity in 9-month-old infants, using event-related potentials (ERPs). Infants were familiarized with an event in the laboratory, then ERPs were recorded as they viewed repeated presentations of pictures of this familiar event and a novel event, along with single presentations of 30 trial-unique events. A middle-latency negative component was sensitive to degree of novelty, differing in amplitude and latency by stimulus condition and across repeated presentations. Long-latency slow-wave activity also related to stimulus condition. Findings have implications for our understanding of infants' detection of novel information and the processes that render the novel familiar. (Contains 5 figures and 3 footnotes.)