Xenotransplantation can facilitate the transmission of pathogenic porcine viruses. The recent transmission of porcine cytomegalovirus/porcine roseolovirus (PCMV/PRV) to the first human recipient underscores the need for more effective screening strategies. Although PCMV/PRV does not infect primate including human cells, it significantly reduced the survival time of non-human primates that received infected pig organs and contributed to the death of the human recipient. To detect PCMV/PRV in both acutely and latently infected animals, a combination of PCR-based and immunological methods is needed. To detect antibodies against PCMV/PRV, a Western blot assay using parts of the recombinant gB protein was established. Similar highly sensitive diagnostic approaches have been developed for numerous porcine viruses, including porcine circoviruses and lymphotropic herpesviruses. These methods are also relevant for the screening of recipients. The effectiveness of these methods was confirmed by screening pigs intended for xenotransplantation, various minipigs, slaughterhouse pigs, and wild boars. While early weaning, colostrum deprivation, cesarean delivery, or embryo transfer can eliminate these viruses from pig herds, porcine endogenous retroviruses (PERVs) pose a unique challenge because they are integrated into the pig genome. PERV-A and PERV-B are found in all pigs and can infect human cells, while PERV-C, which only infects pig cells, is absent in some pigs. Since PERV-C can recombine with PERV-A to form high-titer human-tropic PERV-A/C recombinants, selecting PERV-C-free donor animals is essential. This requires reliable PCR assays capable of detecting all known PERV-C variants. Detecting PERV in xenotransplant recipients is more complex due to microchimerism, also observed in allotransplantation. It is crucial to distinguish between the presence of PERV sequences in disseminated pig cells and a true infection. PCR detection of porcine-specific short interspersed nuclear elements (SINEs), which are abundant in the pig genome, can identify pig cells in the recipient. Using this method, pig cells have been found in all organs of baboons transplanted with pig hearts. Additionally, screening for antibodies against PERV can determine whether infection has occurred. Western blot assays utilizing recombinant core and envelope proteins of PERV, along with corresponding positive control antisera generated by immunizing animals with these proteins, have been developed. To date, no antibodies against PERV have been detected in any of the investigated clinical trials transplanting pig islet cells, in preclinical studies transplanting pig cells and organs into non-human primates, or in experimental infections in small animals and non-human primates. These findings indicate an absence of PERV infection. Since most of these screening methods are unavailable in standard veterinary laboratories, testing must be conducted in specialized virological units.