Abstract: Cloud computing is a promising technology able to strongly modify the way computing and storage resources will be accessed in the near future. Cloud systems offer services at three different levels: infrastructure as a service (IaaS), platform as a service (PaaS), and software as a service (SaaS). In particular, IaaS clouds provide users with computational resources in the form of virtual machine (VM) instances deployed in the provider data center. PaaS and SaaS clouds offer services in terms of specific solution stacks and application software suites Performance evaluation of Cloud Computing. An infrastructure is required to predict and quantify the cost-benefit of a strategy portfolio and the corresponding Quality of Service (QoS) experienced by users. In a market-oriented area, the cloud computing requires an accurate evaluation of these parameters is required to quantify the offered QoS and opportunely manage SLAs. There are several performance metrics have been defined, like availability, utilization, and responsiveness, allowing us to investigate the impact of different strategies on both provider and user point of views. A resiliency analysis is also provided to take into account load bursts. We can also include the analysis of autonomic techniques able to change on-the-fly the system configuration to react to a change on the working conditions. We can also extend the model to represent PaaS and SaaS cloud systems and to integrate the mechanisms needed to capture VM migration and data center consolidation aspects that cover a crucial role in energy saving policies.