The advent of cloud computing is a new opportunity for companies to rely on highly dynamic distributed infrastructures to offer services to their customers. While major companies might prefer to own the infrastructure to have full control on the data and applications, SMEs look with interest to cloud technology since they cannot afford the initial barrier cost to enter the market. In this perspective the advantages of cloud computing are well known: on-demand self-service; broad network access; resource pooling to optimize usage of provider resources; rapid elasticity to dynamically adapt the allocated resources to the customer needs; and measured service to provide performances guarantees and support the pay-per-use model [3]. Although this market is in rapid expansion, this growth may soon be hindered by user concerns such as lock-in within a single commercial offer (which reduces the necessary competition between many infrastructure providers), ownership and privacy issues of the data stored in the cloud, and the lack of performance predictability of current clouds. As such, it is not yet clear whether cloud computing can offer a valid and dependable alternative to private data centers for business applications, given their strong requirements in terms of availability, reliable application execution, and security.