The trend to outsource applications not critical to an enterprise’s core business has driven the emergence of a new type of IT service providers. These service providers run and maintain applications for enterprises having outsourced them. The business model of the providers is thus based on the exploitation of economies of scale by offering the same infrastructure, platforms and applications to multiple customers. Since different customers have different requirements regarding functional and nonfunctional aspects of an application, the infrastructure, platforms and applications must be customizable to different customer’s needs. This also enables the providers to increase the customer base for one offering. To further increase the customer base, providers do not only host and provide software they have developed in house, but also want to offer applications offered by third-party application vendors. In this thesis, a metamodel, algorithms and tools are introduced allowing application vendors to describe and package applications in a way that they can automatically be set up (provisioned) at a provider either with minimal or no human intervention. Furthermore, a metamodel is introduced allowing application vendors to describe the variability of an application. This variability metamodel enables the generation of customization workflows used by customers to adapt an application to their needs. The combination of the application metamodel and variability metamodel enables a self-service model in which customers can subscribe to and unsubscribe from applications as they wish. Customers thus select the application they want to use in an application portal and a guided through the customization. Having customized the application, the application is set up automatically with the required quality of services and functionality. The concepts introduced in this thesis enable application vendors to describe customizable applications without knowing the exact infrastructure they are later provisioned on. Providers can extend their customer base by offering customizable applications developed by application vendors, and customers can follow a best-of-breed strategy in choosing from arbitrary combinations of providers and applications. Corresponding prototypes for all three roles, application vendors, providers and customers to build, provide, provision and customize applications have been built and are introduced in this thesis. These serve as a proof-of-concept implementation for the proposed concepts. Different case studies are given to show the general applicability of the approach.