Xenotransplantation using pig cells, tissues or organs might be a promising solution to overcome the shortage for organs suitable for allotransplantation. Because of several reasons, the pig is currently the favoured donor species. However, the use of porcine xenotransplants is associated with the risk of transmitting porcine viruses to the human xenotransplant recipient. Among them porcine endogenous retroviruses (PERVs), porcine cytomegalovirus (PCMV), porcine lymphotropic herpesviruses (PLHVs), porcine circovirus 2 (PCV2) and hepatitis E virus (HEV) play a role. Some of them cause immunosuppression and a zoonotic potential of others has been supposed. Therefore the possibility of direct transmission of those viruses between pigs and humans might be possible. Strategies to avoid the transmission of those pathogens are currently of main importance to increase lifetime of the transplant and therefore to save many lives of people standing on the transplant waiting list. To select virus-free animals as putative donor pigs and to recognise transmission of pathogens to transplant recipients, sensitive detection methods are needed. In this study the prevalence and expression of these selected viruses should be investigated and assessed in order to obtain safe and healthy donor pigs for xenotransplantation studies. Therefore highly sensitive PCR-based methods, real-time PCR and real-time RT-PCR specific for all the viruses listed above, as well as immunological methods measuring virus-specific antibodies by Western blot analysis or ELISA were developed. Recombinant viral proteins were cloned, expressed and chromatographically purified as well as purified virus particles were expanded to be used as antigens. The methods were developed and optimized to screen (i) Göttingen minipigs, a well characterized pig breed which is kept in a specific-pathogen free facility, (ii) Aachen minipigs, a pig breed existing since 2013, (iii) slaughterhouse pigs from a butchery in the north of Berlin and (iv) multiply genetically modified pigs produced especially for xenotransplantation. Human-tropic PERV-A and PERV-B were found in all pigs and pig-tropic PERV-C and recombinant PERV-A/C were found in many pigs. HEV, PCMV, PLHVs and PCV2 were found in a few animals. No transmission of the porcine viruses listed above was observed during the transplantation of genetically modified islet cells into four marmosets. However, when transgenic pig hearts were transplanted into baboons, then PCMV and HEV were found transmitted, despite the fact that the donor pigs were negative when testing blood and antibody response. To avoid future transmissions of porcine viruses, more sensitive detection methods, different time points of testing, and different source materials, including oral and anal swabs, should be used. In the study sensitive and reliable methods for the detection of porcine viruses were developed and those viruses were detected in all tested pig herds. Furthermore, potentially zoonotic viruses like HEV and viruses causing immunosuppression like PCMV, PLHVs and PCV2 are present in pigs for slaughter. Although the expression of these viruses were low, the meat-producing and -processing industry should be aware of the improvement of hygienic standards. The newly developed detection methods are a prerequisite for the selection of virus-free pigs for transplantation trials as well as elimination programs based on treatment, vaccination, Caesarean delivery, early weaning and embryo transfer.