Since lung is a directly affected organ by cigarette smoking, various respiratory diseases including lung cancer, chronic obstructive pulmonary disease, interstitial lung diseases, bronchial asthma, are caused and worsen by cigarette smoking not only in case of active smoking but also in case of passive smoking. A lot of carcinogen in cigarette smoke causes lung cancer through the DNA damage. Oxidants in cigarette smoke induce airway inflammation and tissue injury. Various kinds of protease including neutrophil elastase and matrix metalloproteinase cause emphysema. Meanwhile, inflammation also induces lung fibrosis. Cigarette smoking is a risk factor for the development of asthmas and is associated with decreased asthma control and increased risk of mortality and exacerbations.