This dissertation develops a model-based framework and decision support system to help decision makers design free-floating carsharing systems for different cities. It identifies the key aspects that need local adaptation as the operating area, fleet size, charging concept, fleet mix, and price. Models are developed for each of these aspects based on factors like population density, housing costs, distance to city center, and availability of charging infrastructure. The models can be applied through the decision support system to plan free-floating carsharing systems tailored to individual cities. The framework was demonstrated in a case study planning a system for Chicago.