One element of the Safe Drinking Water Act amendments is a wellhead protection program. Wellhead protection areas are defined in the SDWA as "the surface and subsurface areas surrounding a water well or wellfield, supplying a public water system, through which contaminants are reasonably likely to move toward and reach water well or wellfield." Criteria on which wellhead protection areas may be delineated may include distance, drawdown, travel time, and flow system boundaries. Mathematical simulation models often provide the only method to determine the wellhead protection area when quantitative criteria are used. This paper describes models for wellhead protection area delineation, the model selection process, selection criteria, information sources, and research needs. The model must be chosen based on the type of problems involved in the wellhead area: for example, flow or solute transport. Selection criteria include availability of the model, documentation, user support, usability, reliability, and the use of the model (i.e., it's popularity).