Preface In order to better understand clinical issues in pulmonary medicine, it has become obvious that knowledge about structural changes in the lungs has to be improved. The lungs constitute the largest contact area of the human body with the environment and surroundings. In addition, the lungs serve as a capillary filter of venous blood, involving heavy exposure to the external and internal environment. This poses a unique situation in which both exposure to external agents and "endogenous" changes have to be considered when evaluating pathological changes of the lungs. Therefore, a close collaboration between pathologists and clinicians is of great importance in diagnosing and treating all disorders displaying alteration of lung structure. It is, therefore, a pleasure to introduce the current issue of the European Respiratory Monograph, which is dedicated to lung pathology. The Monograph covers the pathology of neoplastic diseases, infections, obstructive and interstitial lung diseases, and pulmonary manifestations of systemic diseases, with all the chapters written by distinguished experts. It is my firm belief that this Monograph will serve as a tool for clinicians to better understand lung pathology. It will also be useful to clinicians who meet patients with pulmonary diseases. As it is a comprehensive update of the pathological field I am assured that pathologists and scientists within the field, will also find this Monograph helpful. K. Larsson Editor in Chief Eur Respir Mon, 2007, 39, vii. Printed in UK - all rights reserved. Copyright ERS Journals Ltd 2007 European Respiratory Monograph ISSN 1025-448x. vii