Cloud computing is storing data/applications on remote servers, processing them from servers, and accessing them via internet. Rather than managing files on local storage devices, cloud computing makes it possible to save them over the internet and access them from the internet by a third party “cloud provider” to provide the clients with services anywhere, at any time. In order to provide cloud resources, cloud computing employs virtualization and resource provisioning techniques. Optimizing resources in the cloud is a main benefit, minimizing cost and satisfying client requests are the goal. In this paper, many resource allocation strategies and their challenges are presented.