Illness and injury are physiologic stressors that alter the body's metabolic and energy demands. Approximately 30 to 55% of hospitalized patients have evidence of malnutrition, which makes nutrition screening and assessment an integral part of the evaluation of the critically ill adult. Nutritional assessment relies on a complete history and physical examination, appropriate laboratory measurements, and diagnostic testing as warranted. Although a single laboratory result may be helpful for nutritional screening, there is no single parameter that is both sensitive and specific for the diagnosis of malnutrition. Instead, nutritional assessment must be ongoing and be derived from a variety of sources in order to identify nutritional trends over time. Early identification and nutritional intervention can lessen morbidity and mortality risks; however, underlying acute and/or chronic disease processes often need to be identified and corrected before the body can reverse abnormal nutrient metabolism. A comprehensive nutritional assessment, incorporated with clinical status, will provide the basis for a nutritional support plan and evaluation strategies. In order to help the advanced practice nurse determine the appropriate nutritional regimen, this article discusses the importance of the patient history, physical examination, body composition measurement techniques, and laboratory data assessment.