Chronic lung allograft dysfunction (CLAD) is a major limitation to long-term success of lung transplantation. Restrictive allograft syndrome (RAS) is a recently discovered subtype of CLAD, showing distinct clinical, pathological and radiological features compared with the major CLAD subtype, bronchiolitis obliterans syndrome (BOS). Introduction of the novel CLAD classification system that differentiates CLAD into BOS and RAS has stimulated research activities aiming delineation of the underlying pathological mechanism in the 2 CLAD subtypes. Accumulating evidences suggest that BOS and RAS could be biologically distinct, indicating that optimal therapeutic strategy could be different. Further efforts in refinements in biologic profiling of BOS and RAS will lead to a better understanding of CLAD, as well as the development of personalized and targeted preventive therapies for underlying CLAD subtypes.