The growing number of commercial and scientific clouds strongly suggests that in the near future users will be able to combine cloud services to build new services, a plausible scenario being the case when users need to aggregate capabilities provided by different clouds. In such scenarios, it will be essential to provide virtual networking technologies that enable providers to support crosscloud communication and users to deploy crosscloud applications. This chapter describes one such technology, its salient features and remaining challenges. It also makes the case for crosscloud computing, discussing its requirements, challenges possible technologies and applications.