There exist two principally different approaches to the organization of problem solving in VO: (i) information resources driven approach (choice and integrated definition of resources are made independently of the problem specification); and (ii) scientific problem driven approach (a specification of a problem domain is created, the relevant to the problem resources are identified and semantically mapped into the domain). Intrinsic difficulties of the first approach: semantic gap between resources and the problem, instability of global schema w.r.t. a set of resources, inability of automatic identification of resources for the problem. To implement the second approach a mediation technology is required. On the consolidation phase of the mediator the efforts of the scientific community are focused on the problem definition by specifying the mediator. During the operational phase relevant information resources are identified and expressed in terms of the mediator. Advantages of the mediator approach include truly semantic integration of heterogeneous resources due to their semantic mapping into the mediator; multiple subjects can be semantically integrated applying recursive structure of the mediators.