In 1990, an international grading scheme for the grading of pulmonary allograft rejection was instituted. The use of this classification has resulted in a uniformity of grading which has allowed inter-institutional collaborations and communication unique in allograft monitoring. In 1995 an expanded group of international pathologists convened and revised the original proposal. This article summarizes the updated classification for pulmonary allograft rejection. In brief, acute rejection is based on perivascular and interstitial mononuclear infiltrates. Each grade of acute rejection should mention the presence of coexistent airway inflammation, the intensity of which may also be graded. Chronic rejection is divided into bronchiolitis obliterans--active or inactive--and vascular atherosclerosis--accelerated arterial or venous sclerosis.