I see cloud computing as a natural evolution of concepts in Grid computing. We/I (globus) have been working on mechanisms for lease-based resource provisioning (as opposed to job management with resource provisioning treated as a side-effect) for a long time now: this investigation was motivated by grid computing scenarios and applications and targeted at solving specific problems that Grid users experienced. There is nothing conceptually different between the workspace service developed as a result (which Sergio references above and which I lead) and EC2 although there probably is a difference in the background that motivated them. Whether resources are provided immediately/real-time, as an advance reservation, or on a best-effort preemptbible basis is a property of a resource lease you choose to provide (the terms of service on a lease) but not a fundamental difference. Similarly, the size of a lease that you choose to provision is just another difference in terms. In the science clouds (http://workspace.globus.org/clouds/) we run at University of Chicago and Florida we get requests for leases of all shapes and sizes, wide and narrow, longer-term mixing happily with short-term and I would say all of our users right now come from the Grid community. And we can easily migrate them to EC2 precisely because the two services are nearly the same. Having said all that -- while I see cloud computing as an evolution of grid computing I do think it is a *significant* evolution. The focus on resource leases enabled by virtualization (what Thorsten I think refers to as "forking the server") rather than scheduling jobs is a fundamental difference. The notion that you can create a custom environment and map it onto resources in an easily relocatable fashion will refactor the roles in what are now called grid communities. Five, ten years down the road I don't think we will see any distinctions.