The diagnosis of brain death is based on a comprehensive neurological evaluation. First, physicians need to eliminate confounders to the clinical examination. Once excluded, a set of neurological tests and a formal apnea test (to document absent respiratory drive after CO2 challenge) often will suffice. Second, ancillary tests may be needed if some parts of the neurological examination cannot be accurately assessed. This chapter provides a full discussion of the clinical criteria in adults and children and is modeled after the guidelines of the American Academy of Neurology and the 2011 definition of pediatric brain death by a joint task force of the Society of Critical Care Medicine, the American Academy of Pediatrics, and the Child Neurology Society.