Governments and public sector agencies are continuously looking for ways to improve their services. Therefore, there is a need for restructuring processes and effectively using technology to improve efficiency and effectiveness of the business operations. Cloud computing is one of recent technological trends that support these efforts. It is a new type of sourcing model in which computing services are provided as a utility over the Internet. This paper analyzes the benefits and challenges of cloud computing over e-government systems. It reviews the existing literature on the proposed models of cloud computing adoption in the context of e-government. Further, this paper critically analyzes and classifies these models to different categories.