This chapter introduces and describes the process of medical evaluation, also termed medical screening, of the psychiatric patient in the emergency department (ED). It discusses the diagnosis of medical mimics, along with the utility of both the patient history and physical exam and laboratory evaluations. The evaluation that an emergency physician conducts is an extremely important and, albeit, limited chance for the patient to be treated for a medical condition that may be causing their symptoms. The chapter also discusses the use of standard screening algorithms, which have been shown in several studies to decrease testing costs for ED patients undergoing medical screening. Local processes, such as coordination of care, trust between providers, wait times for subsequent psychiatric admission, facility overcrowding, and subgroup demographics may play a strong role in acceptance and accuracy of the emergency medicine evaluation process.