The Cloud computing offers a distributed system over a network in which a program or any application run on many connected computers at the same time. Cloud computing is a hosted service in which an end user can access the cloud based applications through the browser or any mobile application. Though the cloud computing is a very vast and useful technology but there are still some challenges to be solved. Among all of these there are security issues. The security is most important impact of any software or any hardware. So this paper focuses on the security issues arising from the usage of Cloud services and security issues in different service models of cloud computing.