"Software as a Service" (SaaS) is a software distribution model that uses the Internet to deploy, maintain and run software solutions. Applications that are built to be used by thousands of customers have the need to be configurable and customizable to a high degree. This has a strong impact on the applications' architectures. A single code base and a limited number of deployed instances have to serve a large number of customers (=tenants) although the users' view on the system may be very different. In this session Rainer Stropek presents challenges that ISVs are typically faced with when building such configurable multi-tenancy solutions. Based on this discussion Rainer will point out important consequences of multi-tenancy on operational costs and pricing models in SaaS solutions.