Enterprises today constantly seek to reduce the cost of their IT-operations. One popular approach is to outsource (parts) of these IT-operations. With the advent of Cloud computing, the outsourcing of infrastructure, runtime platforms and even whole applications has been greatly facilitated. With platforms such as Amazon’s EC2 or Salesforce’s Force.com customers can select from a set of pre-defined machine images or applications that they can then run on-demand. However, all these platforms employ proprietary technology that does not permit to model, offer, configure and automatically deploy applications in the cloud in a generic manner. In this paper we introduce metamodels, algorithms and tools for application vendors to describe composite service-based cloud applications independently of the provider that later offers them. We describe how customers can customize such service-based applications and how providers can automatically provision the individual services required by the application, thus offering customized applications to their customers while minimizing costs by sharing services and infrastructure across customers. We report on the architecture and implementation of our prototype and show sample applications.