Organ transplant candidates should be optimally screened before transplantation and latent and chronic infections treated. Vaccination status should be reviewed to ensure full compliance with the recommended vaccinations schedule. After transplantation, immunosuppressive therapy increases the risk for infectious complications. An aggressive diagnostic approach is mandatory and must weigh the time elapsed after transplantation as well as the net state of immunosuppression. Principles of establishing a differential diagnosis in transplanted individuals suspected of having an infectious complication are presented. Furthermore, common opportunistic infections and current prevention strategies are discussed.