In this study, we are learning about the dynamic brain activity patterns associated with delirium, a syndrome of sudden onset disorder of arousal and confusion. We recruit patients upon admission to the intensive care unit (ICU), obtaining informed consent from the patient where possible and from their authorized surrogate if necessary. We record clinical EEGs at enrollment and at ICU discharge. We compare the EEG signatures to contemporaneous clinical assessments using the Confusion Assessment Method for the ICU (CAM-ICU), and to formal cognitive assessments performed at 3 months and 12 months after hospital discharge. In 2021, we developed and published an EEG-Delirium Index (EEG-DI) based on the pilot data for this study. The main study, which seeks to validate the EEG-DI in a prospective cohort, has recruited more than 170 participants so far. This study is supported by a K23 award from the National Institute of Aging.