The problems in software cost management are well known. Cost estimates are too low. Software development projects frequently have cost overruns, which are due to poor estimates. A fair amount of work have been done toward developing cost estimation models. These models vary in their outputs (e. g., total cost, manning schedule) and in the factors used to calculate their estimates. They also vary with regard to the type of formula, parameters, use of previous data, and staffing considerations. This paper will distinguish them by the type of formula they use to calculate total effort and staffing level, and will discuss enough models to demonstrate the characteristics of each model category.