Aging populations, limited budgets, changing public expectations, new technologies, and the emergence of new diseases create challenges for health care systems as ways to meet needs and protect, promote, and restore health are considered. Traditional planning methods for the professionals required to provide these services have given little consideration to changes in the needs of the populations they serve or to changes in the amount/types of services offered and the way they are delivered. In the absence of dynamic planning models that simulate alternative policies and test policy mixes for their relative effectiveness, planners have tended to rely on projecting prevailing or arbitrarily determined target provider—population ratios. A simulation model has been developed that addresses each of these shortcomings by simultaneously estimating the supply of and requirements for registered nurses based on the identification and interaction of the determinants. The model’s use is illustrated using data for Nova Scotia, Canada.