Human error (HE) has been implicated in 70-80% of civil/military aviation accidents. Yet, most accident reporting systems are not designed around any theoretical framework of HE. As a result, most accident databases are not conducive to traditional HE analysis, making the identification of intervention strategies onerous. A general HE framework is required around which new investigative methods can be designed and existing accident databases restructured. A comprehensive human factors analysis and classification system (HFACS) has recently been developed to meet those needs. The HFACS framework has been used successfully within the military, commercial, and general aviation sectors to systematically examine underlying human causal factors and improve aviation accident investigations. This paper describes the development and theoretical underpinnings of HFACS in the hope that it will help safety professionals reduce the aviation accident rate through systematic data-driven investment strategies and the objective evaluation of intervention programs.