During winter holidays, most ski resorts expect a lot of skiers, who then congest the ski lifts in the corresponding ski areas. This article presents the first full multi-agent model of skiers in a ski area. Our purpose is to offer a decision-making tool for the operator and design engineering. Supply is represented in a graph of the ski area with slopes and ski lifts. Demand is addressed in a disaggregated way based on users' skiing abilities and the number of skiers per group. Each group moves around autonomously within the network. Three sub-models structure the agents' behavior. The choice model defines the itinerary of each agent, the queue model manages ski-lift congestion and the return journey model assists users in returning to their original point before the closing time of the ski lifts. A software has been implemented and a case study on a large ski area, La Plagne, has been performed to illustrate the effect of the replacement of a chair lift on the whole ski area.