On-Orbit Servicing refers to activities of refueling or repairing space objects like, for example, space satellites while these space objects are in outer space. 

Thereby one can differentiate in on-orbit servicing within manned and within un-manned missions. We have had for example such a mission in the Skylab program in 1973 or in the Saljut 7 program of 1995 and the reparation of the Hubble Space Telescope which was in orbit on mission between 1993 and 2009. Also space stations like Mir or the International Space Station (ISS) needed constant maintenance whilst being in outer space. 

Within the context of un-manned missions there are as well several of such activities. Several concepts serve for example to restore disintegrated satellites. So far, these plans have not reached concrete realization. 

One can within the on-orbit servicing missions differentiate into the tanking, orbital maneuvers, reparation, upgrade, establishing of larger structures and, very importantly for the future, the cleaning of outer space from orbital debris. For all these on-orbit servicing activities the place of a satellite or space object, its status, the communication and the structure of the mission is of importance.

Overall on-orbit servicing is of great importance since it enables to reduce mission costs and thus increases the value of a mission and its flexibility. Since the overall trend of modern space flight is to reduce costs on-orbit servicing will certainly increase in the future.