This lecture introduces Geographic Information Systems (GIS) and their application in participatory planning through case studies in Thies and Seychelles. It covers topics such as territorial dynamics, water management issues, urban development challenges, and the role of GIS in communication and decision-making. The presentation delves into the complexities of data management, visualization, and the importance of user-friendly interfaces. Lessons learned include the significance of simplicity, user customization, interactive tools, and the integration of GIS with internet platforms for sustainable planning.