Cloud computing gives an opportunity to use Internet-based services by business organizations. Use of cloud infra structure will reduce start-up costs, lower capital expenditures for organizations. Organizations use cloud services on a pay-as-you-use basis, access applications only as needed, and quickly reduce or increase capacities. Security is a major concern for data stored on cloud.Security to the data and applications is provided by authorization and authentication. Authorization for accessing the data is provided by access control models.Traditional access control models are not sufficient to cater applications running on cloud due to its dynamic nature. Various attribute based access control models are proposed for cloud computing. This book discusses the various authorization security challenges and proposed attribute based access control models to ensure security. The authors discuss the extremely challenging topics of data ownership, privacy protection, majorly focuses on encryption based access control models. This book is mandatory reading for research scholars, students involved in security aspects of cloud computing.