Chronic traumatic encephalopathy (CTE) is a latent neurodegenerative disease associated with repetitive minor head trauma, including concussion and subconcussion. CTE was first recognized in boxers nearly a century ago as “dementia pugilistica” but has been recently identified in athletes who played a variety of contact sports, including American football, ice hockey, soccer, baseball, rugby, boxing, and wrestling as well as in military veterans exposed to blast and concussive injuries. Like many other neurodegenerative diseases, CTE is diagnosed with certainty only by neuropathological examination of brain tissue. Significant advances have been made over the past decade in classifying and characterizing the neuropathological and immunohistochemical features of CTE. CTE is a tauopathy; it is characterized by the accumulation of hyperphosphorylated tau (p–tau) as neurofibrillary tangles (NFTs), astrocytic inclusions, and dot–like neurites in a unique pattern around small blood vessels in the cortex with a striking tendency to occur in clusters at the sulcal depths. Recently, a consensus panel of expert neuropathologists refined the neuropathological criteria for the diagnosis of CTE, defined a pathognomonic lesion, outlined supportive but nonspecific pathological features, and recommended a minimum blocking and staining scheme for the pathological evaluation for CTE. Symptoms of CTE include behavioral and mood changes that often begin in middle age. In advanced disease, memory loss, executive dysfunction, cognitive impairment, and dementia are common. Most individuals develop symptoms years after exposure to trauma although 20% develop symptoms while still active in sports. Many fundamental questions remain to be answered regarding CTE; we do not know how common it is, and we do not know if there are genetic risk and susceptibility factors. Multicenter, prospective research efforts are currently underway to develop operational biomarkers for CTE and to improve the sensitivity and specificity of the clinical diagnostic criteria. Additional large–scale, longitudinal, epidemiological, genetic, and animal studies will be required in order to develop effective ways to diagnose and treat the disorder during life.