Crack growth rate parameters of the Paris equation are crucial inputs in the engineering critical assessment (ECA) of structures containing flaws. In fracture mechanics based reliability analysis, probabilistic models of these parameters are often used. Despite the considerable body of research in this area, there is significant variability among available models. This paper reviews the current available models in the literature and addresses areas requiring further research with a view to assisting probabilistic flaw assessment. The effect of the existing variability in crack growth model parameters is investigated by fracture mechanics analysis of a case study crack.