An important issue faced by Software-as-a-Service (SaaS) application is scalability. Each SaaS application is typically shared by multiple (tens or hundreds) organizations (tenants). Each tenant may have hundreds or thousands of users. Thus, the number of concurrent accesses is high. Handling a large number of user requests effectively is critical for SaaS applications. Various aspects of SaaS can have a significant impact on its scalability, including levels of scalability mechanisms, automated migration, tenant awareness, workload support, fault-tolerance and recovery, software architecture and database access. This paper identifies scalability factors and discusses their impacts on the scalability of SaaS applications. Existing approaches for addressing the scalibility of SaaS applications are also analyzed, and this paper suggests some alternatives to improve SaaS scalability based on the factors identified.