More and more companies are offering their software by following the Software as a Service (SaaS) model. The promise of the SaaS model is to exploit economies of scale on the provider side by hosting multiple customers (or tenants) on the same hardware and software infrastructure. However, to attract a significant number of tenants, SaaS applications have to be customizable to fulfill the varying functional and quality requirements of individual tenants. In this paper, we describe how variability modeling techniques from software product line engineering can support SaaS providers in managing the variability of SaaS applications and their requirements. Specifically, we propose using explicit variability models to systematically derive customization and deployment information for individual SaaS tenants. We also demonstrate how variability models could be used to systematically consider information about already deployed SaaS applications for efficiently deploying SaaS applications for new tenants. We illustrate our approach by a running example for a meeting planning application.