Obliterative bronchiolitis (OB) is the histologic correlate of chronic allograft dysfunction in pulmonary transplantation. The histologic diagnosis of OB is challenging, therefore a physiologic definition, bronchiolitis obliterans syndrome (BOS) based on pulmonary function tests has been used as a surrogate marker for OB for the last decade. BOS has proven to be the best available surrogate marker for OB and is predictive of the ultimate endpoints of graft and patient survival. Multiple other clinical markers have been reported and proposed as alternates for or complements to BOS grade, but all need further evaluation and validation in large, prospective clinical trials. Lastly, given the early occurrence and high incidence of chronic allograft dysfunction, the easily measurable endpoint of BOS grade, and our lack of understanding of ways to prevent or alter the course of BOS, lung transplant recipients represent an ideal population for clinical trials targeting prevention and treatment of chronic allograft dysfunction.