Modern science is increasingly dependent on information and communication technologies, analyzing huge amounts of data (in the terabyte and petabyte range), running large-scale simulations requiring thousands of CPUs, and sharing results between different research groups. This collaborative way of doing science has led to the creation of virtual organizations (VOs) that combine researches and resources (instruments, computing, data) across traditional administrative and organizational domains [1]. Advances in networking and distributed computing techniques have enabled the establishment of such virtual organizations and more and more scientifi c disciplines are using this concept, which is also referred to as “grid” computing [2-4].