A special interest group of the American Congress of Rehabilitation Medicine [ACRM; Mild Traumatic Brain Injury Committee. (1993). Definition of mild traumatic brain injury. Journal of Head Trauma Rehabilitation, 8 (3), 86-87.] was the first organized interdisciplinary group to advocate four specific criteria for the diagnosis of a mild traumatic brain injury (TBI). More recently, the World Health Organization (WHO) Collaborative Center Task Force on Mild Traumatic Brain Injury [Carroll, L. J., Cassidy, J. D., Holm, L., Kraus, J., & Coronado, V. G. (2004). Methodological issues and research recommendations for mild traumatic brain injury: the WHO Collaborating Centre Task Force on Mild Traumatic Brain Injury. Journal of Rehabilitation Medicine, (Suppl. 43), 113-125.] conducted a comprehensive review of the definitions utilized in evidence-based studies with mild TBI patients. Based on this review, the WHO task force maintained the same four criteria but offered two modifications. The similarities and differences between these two definitions are discussed. The authors of the ACRM and the WHO definitions do not provide guidelines or specific recommendations for diagnosing the four criteria. Thus, we provide recommendations for assessing loss of consciousness, retrograde and post-traumatic amnesia, disorientation and confusion as well as clarification of the neurologic signs that can be indicative of a diagnosis of mild TBI. Finally, confounding factors mentioned in both definitions that should exclude a mild TBI diagnosis are summarized.