This article reviews the persisting difficulty and the importance of the diagnosis of minor head trauma. The diagnosis has been complicated by pervasive disagreement regarding diagnostic criteria. This is primarily a result of the fact that evidence for actual injury is hard to obtain in minor cases because most symptoms tend to be subjective and have high base rates in the normal, uninjured population. At the same time, the diagnostic decision has important implications for patients in terms of treatment, expectancy for future function and lifestyle, and compensation for injuries. Decision theory leads us to the awareness of diagnostic errors. In addition to correct determination, the clinician can make an error of not diagnosing an injury when it has in fact occurred or making a positive diagnosis where there is no injury. The optimal strategy is to set the cutoff at the midpoint of these two error probabilities. The clinician may be willing to make one error rather than the other depending on the cost and bias involved. The second error is more likely to be made when the clinician stands as a strong advocate for the patient and willing to provide any help necessary to encourage treatment, give patients a rationale for understanding their symptoms, and help them obtain compensation for injuries. This can also lead to significant overdiagnosis of injury. The first error is more likely to be made when the clinician recognizes the potential for increasing costs to the health-care industry, the court system, and increasing personal injury claims. He or she may also recognize the vulnerability to the risk for symptom invalidity, the perpetuation of patient symptoms through suggestion, and the need for a biologic explanation for life stressors and preexisting emotional and personality constraints. It can be argued that the most objective diagnostic opinion, uninfluenced by the above biases, should ultimately be in the best interest of the patient, the clinician, legal consultants, and society. Based on the findings in this chapter, at least four symptom constellations can be identified. These have differing probabilities for residual symptoms of minor head trauma and include the following: 1. These patients' symptoms clearly meet the criteria from Table 2. This includes several findings from 1 to 10 of Table 1, together with abnormal neuropsychologic testing on the AIR, General Neuropsychological Deficit Scale, or other indicators of diminished cortical integrity. This group of patients shows a very strong probability of having experienced a brain injury and for showing residual symptoms of minor head trauma. 2. These patients have experienced concussional symptoms (e.g., headache, mild confusion, and balance and visual symptoms) that were documented at the time of injury but sustained no or brief (< 15 seconds) LOC or PTA and, therefore, do not qualify for the diagnosis in Table 2. They may still have several symptoms from Table 1, including objective findings from neuroscanning and variable neuropsychologic testing, especially in measures of attention and delayed recall. This group also shows a high probability for residual, unresolved concussional, and related symptoms. 3. These patients may have shown evidence of concussional symptoms at the time of injury, with no or brief LOC, PTA, or other symptoms from Table 1 (1-10). They continue to show persistent symptoms after 6 months to 1 year. With this group, there is a strong probability that emotional, motivational and premorbid personality factors are either causing or supporting these residual symptoms. 4. In these patients, clearly identifiable postconcussive symptoms at the time of injury are not easy to identify, and perhaps headache is the only reported symptom. There was no LOC or PTA, and virtually none of symptoms 1 to 10 in Table 1 are observed. These patients show strong evidence of symptom invalidity on MMPI-2 or other measures, and marked somatoform, depression, anx