A leading complication of leukemia therapy and stem cell transplantation is opportunistic viral infections. Infections caused by cytomegalovirus, herpes simplex, varicella-zoster, Epstein-Barr, and the community respiratory viruses are associated with significant morbidity and mortality in this highly immunosuppressed population. Fortunately, a growing armamentarium is allowing more effective prophylaxis of these pathogens. This article reviews the epidemiology and prophylactic strategies available for these common opportunistic viral pathogens.