Plant pathology exists as a separate science to a large extent because plant diseases cause crop losses. Paradoxically, few reliable estimates of crop loss in relation to disease exist. Crop loss is defined as the measurable reduction inquantity and quality of yield. Economic loss is an additional subject that is influenced by a series of variables not exclusively related to the crop and disease in the field. Economic loss is not discussed further. Crop loss in its entirety is a function of disease, insects, weeds, other pests and environment. For phytopathology, crop loss can be considered a function of the disease epidemic plus an "error term," where the error term accounts for the unexplained variability. Epidemic characteristics include: final level of disease (X(f)), initial amount of disease (X_o), time of epidemic onset (t_o), rate of increase at any time (r), and area under the disease progress curve (ADPC). Models for crop can be categorized as those using single (critical point) or multiple (multiple point) edidemic characteristics. Over half of all disease loss models use only a single epidemic characteristic.