In the 20th century, chronic traumatic encephalopathy (CTE) was conceptualized as a neurological disorder affecting some active and retired boxers who had tremendous exposure to neurotrauma. In recent years, the two research groups in the United States who have led the field have asserted definitively that CTE is a delayed-onset and progressive neurodegenerative disease, with symptoms appearing in midlife or decades after exposure. In the modern descriptions of post-mortem autopsy cases of former boxers and football players, between 2005 and 2012, the neuropathology attributed to CTE was broad and diverse. This diverse pathology, arising from multiple causes, was aggregated and referred to, in toto, as the pathology “characteristic” of CTE. Preliminary consensus criteria for defining the neuropathology of CTE were published in 2016. Most of the neuropathology described as characteristic of CTE in studies published before 2016 was not included in the 2016 research criteria for defining the pathology of CTE. In 2021, revised consensus criteria for defining the neuropathology of CTE were published. There are no validated criteria for diagnosing CTE, or traumatic encephalopathy syndrome, in a living person, and using the leading research criteria for clinical diagnosis, published in 2014, can result in very high false positive diagnostic rates. Consensus criteria for diagnosing traumatic encephalopathy syndrome, the clinical syndrome hypothesized to be associated with CTE neuropathology, were published in 2021 and will be discussed.