An original methodological approach for forensic diagnostics of diffuse axonal injury (DAI) was developed based on the comprehensive study results. The approach is based on the algorithm of expert actions, including utilizing the developed rational methods set to identify pathognomonic morphological features using accessible and effective histological techniques. Also, the approach includes ways of analysis and estimation of these features. The proposed methodological approach aims to provide an objective diagnosis of this type of traumatic brain injury (TBI) and establish the age of its acute post-traumatic period. The known and generally accepted definitions of DAI and TBI are clarified.