Fever is a common clinical manifestation in transplant patients, and it may be due to many different reasons. In the general population, infections and malignancies as causes of fever of unknown origin (FUO) have decreased over time, whereas inflammatory diseases and undiagnosed fevers have increased. This article reviews FUO in transplant patients, despite there being no clear and widely accepted definition of FUO for such patients. The methodology of diagnosis has been accelerated and made more accurate, not only for the imaging diagnosis but also for microbiology. With respect to diagnosis of FUO in the transplant patient population, consideration should be given to the specific type of transplant patient, to the results of the physical examination, and to the epidemiological antecedents. This article examines an approach that considers different syndromes, followed by an etiologically oriented differential diagnosis.