Differential diagnosis is at the heart of every initial clinical encounter and is the beginning of every treatment plan. The clinician must determine which conditions are possible candidates for consideration and then choose from among them the disorder that bests accounts for the presenting symptoms. The biggest problem encountered in differential diagnosis is the tendency for premature closure in coming to a final diagnosis. A number of studies have indicated that clinicians usually decide on the final diagnosis within the first 5 minutes of meeting the patient. Forming initial impressions can be valuable in helping to suggest which questions need to be asked and which hypotheses need to be tested. Unfortunately, however, often enough first impressions are wrong—particularly because the patient’s current state may not be an accurate reflection of the longitudinal course. Accurate diagnosis requires a methodical consideration of all of the possible contenders in the differential diagnosis. The use of the differential diagnosis tables in this chapter will ensure that your evaluations are systematic and comprehensive.