In the recent years, Software-as-a-Service has gained growing attention from software vendors as well as software customers. In this distribution and pricing model, software vendors are responsible for the operation and maintenance of solutions and customers pay for the service in form of continuous usage-based subscription fees. This study analyses the key characteristics of the Software-as-a-Service concept and evaluates their influence on development processes of software vendors. Based on the literature, two defining characteristics (vendor-hosted, pay-per-usage) and five supportive characteristics (standardization, web-technologies, multi-tenancy, fine-granularity, continuous evolution) are identified. Two vendors of complex business applications – both applying a mix of deployment models – are analyzed to identify the impact of Software-as-a-Service on development processes, as well as the driving forces behind the change. The results indicate, that the concept especially affects the requirements engineering as well as operations phases. The combination of the defining characteristics results in an increased cost as well as innovation pressure. Vendors are challenged to optimize and streamline existing practices to reduce internal costs. At the same time, vendors need to innovate quicker. The combination of these aspects asks for an increased integration of internal activities like development and operations, as well as an increased customer-orientation and integration.