Abstract: Allogeneic hematopoietic stem cell transplantation (allo-SCT) is an effective therapy for many hematologic nonmalignancies or malignancies. In long-term follow-up after transplantation, graft versus host disease (GVHD), with multiple systems involved, remains a major cause of non-relapse mortality. In the past few years, pulmonary GVHD has drawn more and more attention, with bronchiolitis obliterans syndrome (BOS) as the most common one. BOS results in progressive circumferential fibrosis and cicatrization of the ultimate small terminal airways, which is associated with high mortality. This review discusses the clinical characteristics, pathogenesis, predictors, and treatment of BOS.