Understanding accident causation is intrinsic to their successful prevention. To shed light on the accident phenomenon, over the years authors have developed a plethora of conceptual models. At first glance they seem as diverse and disparate as the accident problem they purport to help solve, yet closer scrutiny reveals there are some common themes. There are linear models which suggest one factor leads to the next and to the next leading up to the accident and there are complex non linear models which hypothesise multiple factors are acting concurrently and by their combined influence, lead to accident occurrence. Beginning with a look at the historical context, this chapter reviews the development of accident causation models and so the understanding of accidents. As this understanding should underpin OHS professional practice the chapter concludes with a consideration of the implications for OHS professional practice.