A few technologies have been crucial in enabling the development and use of cloud platforms. Web services allow applications to communicate easily over the internet: Composite applications are easily assembled from distributed web-based components using ‘mashups.’ If there is one technology that has contributed the most to cloud computing, it is virtualization. By decoupling the software platform from hardware resources, virtualization enables massive cloud data centers to function seamlessly in a fault-tolerant manner. Similarly, multi-tenancy allows the same software platform to be shared by multiple applications, and can thus be looked upon as application-level virtualization. Multi-tenancy is critical for developing software-as-a-service applications and Dev 2.0 platforms.