Although transplantation of isolated lung tissue was first attempted in 1963, it was not until 1983 that Cooperet al.performed the first successful single lung transplant operation. Short-term survival has improved with time, but bronchiolitis obliterans hinders long-term success. Bronchiolitis obliterans syndrome (BOS) was the term assigned for allograft dysfunction not explained by acute problems. Although BOS has a variable rate of progression, treatment in general has been unsatisfactory. As an entity, BOS is the single leading cause of death 1 or more years after lung transplant, based on United Network for Organ Sharing data. The incidence of the disease has been reported at 15 to 20% per year in patients posttransplantation, and in up to 75% of surviving patients by 5 years posttransplantation. This common and devastating complication is the target of these reviews. This article will deal with risk factors for BOS and some markers for disease activity that might predict development and progression of the disease.