This slender volume of introductory notes on concussive brain injury (CBI) is published at a curious historical moment. The twenty-first century has blessedly heralded the first serious reconsideration of what a concussion is for perhaps more than 100 years. The eye-opening discoveries that typical CBIs are commonly associated with lasting deleterious effects and that sport-related repetitive head impact may often or always cause brain damage have been met with various degrees of shock, resistance, and hope for progress in human knowledge. However, a Gibraltar-sized stone in the road blocks any major advance. We cannot diagnose CBI in a scientific way. That fact has provoked a costly global search for a valid biomarker -- and perhaps unseemly commercialization of that quest. In 2018, one for-profit corporation (reportedly funded with many taxpayer dollars) announced their discovery of a "concussion test." They found nothing of the sort. The co-authors of the present chapter are widely regarded as the genuine world authorities on potential chemical markers for CBI. Their review counterbalances the overheated commercial rhetoric and explains the prospects for science.