Over the last years many companies and national agencies in different countries have been involved in development of various technical aspects of On-Orbit-Servicing (OOS). US and Russian OOS experiences are described. The problem of OOS in general is considered as a bit wider. It is shown that OOS is connected with development of the Next-Generation Space Infrastructure and the solution of the problem of OOS, to a great extent, predetermines the characteristics of the Next-Generation Space Systems. Two equally important directions are stressed for OOS activities: first, making satellites serviceable, and the second, creating directly servicing systems. Implementation of each direction includes a wide range of developments. In the first case, we have to consider a capability of docking with the serviced satellite, a guaranteed access to the satellite components, block-modular structure of the serviced satellite, standardization of hardware and connectors, etc. Implementation of the second direction varies from the development of servicing methods and servicing systems to satellite orbits and constellation optimization. The existing and perspective key technologies for Serviceable and Servicing Satellite are presented. It is shown, for instance, that the economic benefit of OOS has to be justified by more thoroughly from an end-to-end perspective taking into account the features of the future Space Infrastructure. Servicing allows extending operational lifetime of satellites and thus reducing lifecycle cost, or moreover enable for entirely new systems and mission. These effects could be achieved not only through refuelling or repairing of the satellites, but also through satellite orbit correction. OOS creates a prospect of establishing a commercial servicing and debris removal network lending form the same technology base, which constitutes, however, separate technological problems, which are closely connected with OOS.