How can anesthesiologists tell when patients are unconscious under general anesthesia? Since the 1930's, stereotyped electroencephalogram (EEG) patterns have been observed during general anesthesia, yet the specific signals that demarcate loss and recovery of consciousness have remained elusive. In this talk, I will be presenting results from human studies showing behavioral changes and EEG signatures that occur in the transition between consciousness and unconsciousness during gradual induction and emergence from general anesthesia. We characterized level of consciousness using an auditory response task, and observed that the probability of response decreased gradually in the transitions to and from unconsciousness. The transition to loss of consciousness was marked by increased gamma and beta power that decreased in center frequency and bandwidth as the probability of response decreased. At loss of consciousness, low-frequency and globally-coherent frontal alpha oscillations developed, while occipital alpha oscillations were abolished. During emergence from propofol anesthesia, these EEG patterns reversed. These results establish electrophysiological signatures that can be used to monitor and manage the state of unconsciousness under general anesthesia, and provide important insights into the mechanisms underlying this state.