Human error continues to be implicated in the vast majority of aviation accidents. Yet, most accident investigation and reporting systems are not currently designed around any theoretical framework of human error. One reason may be the discontinuity between classical theories of human error and the practical application of these theoretical approaches in accident investigation. The Taxonomy of Unsafe Operations presented here bridges the gap between theory and practice by providing field investigators with a user-friendly, common-sense framework from which accident investigations can be conducted and human causal factors classified. This taxonomy draws upon traditional models of human error to account for human accident causal factors, including the condition of operators and supervisory error. A detailed description of the taxonomy is provided, as well as a discussion of the benefits it provides the field of accident investigation and prevention.