A promising vision of distributed cloud computing is a unified world of multiple clouds, with business benefits at hand. In practice, lack of interoperability among clouds and management complexity raise many security and dependability concerns. The authors propose secure Supercloud computing as a new paradigm for security and dependability management of distributed clouds. Supercloud follows a user-centric and self-managed approach to avoid technology and vendor lock-ins. In Supercloud, users define U-Clouds, which are isolated sets of computation, data, and networking services run over both private and public clouds operated by multiple providers, with customized security requirements as well as self-management for reducing administration complexity. The article presents the Supercloud architecture with a focus on its security infrastructure. The authors illustrate through several use cases the practical applicability of the Supercloud paradigm.