Agitation is inappropriate verbal, vocal, or motor activity, which is common among older persons with dementia. Its assessment is complicated both by communication difficulties and by the complexity of the manifested behaviors. Three general approaches are used to assess agitation: informant ratings, observational methods, and technological devices. Within each category, several assessment instruments exist. The approaches differ as to time sampling, objectivity, and cost. Assessment of agitation in pharmacological studies should include consideration of the specific meaning to the agitated behavior for the older person and for the caregiver, along with an evaluation of the consequences of the behavior. At times, the agitated behavior seems to signal a specific need, and alternative methods for fulfilling that need should be considered. Evaluation studies of pharmacologic agents should include consideration of the impact of these agents on nontarget behaviors, as well as an awareness of the large intraperson and interperson variability in agitated behaviors.