Résumé : Cloud platforms are increasingly used for hosting a broad diversity of services from traditional e-commerce applications to interactive web-based IDEs. However, we observe that the prolif- eration of offers by Cloud vendors raises several challenges. Developers will not only have to deploy applications for a specific Cloud, but will also have to consider migrating services from one cloud to another, and to manage applications spanning multiple Clouds. In this paper, we therefore report on a first experiment we conducted to build a multi-Cloud system on top of thirteen existing IaaS/PaaS. From this experiment, we advocate for two dimensions of adaptability - design and execution time - that applications for such systems require to exhibit. Finally, we propose a roadmap for future multi-Cloud systems.