The lung is the first target organ which is affected in its structure and function by inhalation of cigarette smoke The lung and the airways are working virtually as a filter for several hundred damaging substances, which are contained in cigarette smoke. As a consequence cigarette smokers are showing a markedly elevated prevalence for development of airway obstruction, and clinical symptoms leading to chronic obstructive pulmonary diseases (COPD) as well as to recurrent respiratory tract infections. As a result smokers are showing an elevated mortality rate caused by lung cancer as well as by development of COPD. The irritating and damaging activity of cigarette smoke to the lung is dose dependent. It is concluded that cigarette smoking is by far the largest preventable risk factor for development of chronic lung diseases.