Infrastructure-as-a-Service (IaaS) clouds are evolving from offering simple on-demand resources to providing diverse sets of tightly-coupled monolithic services. Like OSkernelsofthe1980’sand1990’s,thesemonolithicofferings,albeitrichinfeatures,aresignificantlyconstraining users ’ freedom and control over the underlying— cloud—resources. For example, we are unaware of a true hybrid cloud, where its users can migrate virtual machines freely across clouds. This paper argues for a new type of IaaS cloud, an xCloud, that builds on ideas fromextensible OSsto give usersthe flexibilityto install custom cloud extensions, which can address the limitations outlined above. We describe the design space for xClouds, includingapractical approachfor transforming today’spublic cloudsintoxClouds.