The treatment of genetic diseases and of several acquired diseases can only reasonably be performed by using gene or cell therapy approaches. In principle, practically all viruses can be used as a base for vector systems (→ viral vectors) for the transfer of genetic material/genes into target cells, and in dependence of the viral vector system used the gene is integrated into the cellular genome or stays in an episomal state. The most important viral vectors presently used are retroviral (RV) and adenoviral (AdV) vectors, which are classical vectors, followed by adeno-associated virus (AAV) and lentiviral (LV) vectors, both of which have a very bright future because of their superior characteristics with respect to RV and AdV vectors. Although this chapter does not deal with other vector systems, it should be mentioned that further viral vector systems with other interesting characteristics, such as those vector systems based on herpes simplex virus or α viruses, are in development and will probably be used for future clinical applications. This methods chapter deals with cells and production systems used for producing RV, LV, AdV, and AAV vectors, as well as with a short update on the purification and the titration of the produced viral vectors. With respect to retroviral vectors, the developments leading to optimal and safe (third- and fourth-generation producer cell lines for oncoretro- and lentiviral vector production, respectively) producer cells are presented. Concerning adenoviral vectors, the different vector generations as well as producer cells (from HEK293 to Per.C6) are shown, whereas the various production approaches are briefly described for the AAV vector system (from a transient production system via producer cells to the insect cell/baculovirus production system). Basic titration methods are also presented.