Viral infections are among the most common infectious complications affecting transplant recipients. Due to immunosuppressive therapy predominantly affecting cellular immunity and thus successfully reducing the incidence of acute rejection, there is a higher incidence of viral infections. Herpesviruses and polyomaviruses are ubiquitous pathogens which have the ability to persist in a state of latent infection. In addition to post-transplant reactivation, donor-induced primoinfection can also occur, leading to increased morbidity and contributing to decreased survival of patients. Moreover, there are long-term indirect effects, resulting in an impaired function of the transplanted organs and their premature loss. This article provides a brief overview of infections which have the greatest impact on transplant recipients, i.e. cytomegalovirus and BK polyomavirus, their diagnosis, prevention and treatment.Key words: BK virus - cytomegalovirus - ganciclovir - immunosuppressive therapy - JC virus - kidney transplantation - polyomavirus - polyomavirus-associated nephropathy - preemptive therapy - prophylaxis - valaciclovir.