When a person presents with an apparent acute concussive brain injury, whether at the Emergency Room or the primary care office, the treating physician must make just one life-or-death decision: obtain a CT scan? The recent announcement of a blood test that often detects intracranial hemorrhage has raised the hope of reducing the employment of CTs in some circumstances. But neither that innovation -- nor any review, no matter how systematic and comprehensive, nor any published algorithm -- can point to the right decision for every case. This chapter reviews both the evidence and the proliferation of published clinical guidelines, some of which comport with the evidence better than others. In the end, no clinical data reliably predict CT results. The physician is forced to make a decision under uncertainty. Either decision is risky business. The weight of evidence slightly favors the aphorism: when in doubt, scan.