Mild traumatic brain injury is now claimed to be the signature injury of the Iraq and Afghanistan conflicts. During World War I, shell shock came to occupy a similar position of prominence, and postconcussional syndrome assumed some importance in World War II. In this article, the nature of shell shock, its clinical presentation, the military context, hypotheses of causation, and issues of management are explored to discover whether there are contemporary relevancies to the current issue of mild traumatic brain injury. When shell shock was first postulated, it was assumed to be the product of a head injury or toxic exposure. However, subsequent clinical studies suggested that this view was too simplistic, and explanations soon oscillated between the strictly organic and the psychological as well as the behavioral. Despite a vigorous debate, physicians failed to identify or confirm characteristic distinctions. The experiences of the armed forces of both the United States and the United Kingdom during World Wars I and II led to two conclusions: that there were dangers in labeling anything as a unique "signature" injury and that disorders that cross any divide between physical and psychological require a nuanced view of their interpretation and treatment. These findings suggest that the hard-won lessons of shell shock continue to have relevance today.