Lung transplantation is the ultimate therapeutical approach for the treatment of both children and adults with terminal congenital or acquired lung disease. In contrast to survival rates during the first year following transplantation, the long-term survival for patients after lung transplantation has not significantly improved in the past. In addition to other complications, acute cellular rejection constitutes a major cause for diminished function of pulmonary grafts, and can, among other factors, be causative for chronic rejection (bronchiolitis obliterans syndrome, BOS). In 2006, the International Society for Heart and Lung Transplantation (ISHLT) provided a revised version of the grading system for acute and chronic rejection of pulmonary grafts.