This two-part review provides a critical analysis of the scientific and clinical literature on the agitated brain injured patient. Part 1 reviews nomenclature and classification issues, differential diagnosis, and assessment instruments designed for evaluation of the patient. Pathophysiology and treatment approaches will be discussed in Part 2 in a subsequent issue of the Archives. The review was unfortunately hampered by a lack of consistency in definitions, little scientific study of the neuroanatomic and neurochemical basis for the disorder, few outcome studies, and no randomized controlled treatment trials. Part 1 sets forth an interdisciplinary definition of agitation, establishes a differential diagnostic approach, and describes and critiques the assessment instruments available for clinical evaluation of the agitated patient. Part 2 will address treatment interventions including pharmacological, environmental, and behavioral approaches to this patient population.