There are several causes of lung cancer. Firstly, the most important cause of lung cancer is smoking which includes both active smoker and passive smoker. Additionally, the active smoker is the person who directly smokes any kinds of tobacco and the passive smoker is the one who is exposed to the smoke exhaled by other smokers. Another cause is from genetics and lung diseases. It is said that the person; whose relatives or sibling have had a lung cancer; may have a chance to develop the lung cancer. Moreover, there are many lung diseases that contribute to the development of lung cancer such as tuberculosis, chronic obstructive pulmonary disease and chronic bronchitis and emphysema. Lastly, the environmental factor is also the cause of lung cancer. For example, the workers closely connect to asbestos fibers and similar materials may have a five times higher risk of developing lung cancer. In conclusion, it clearly shows that lung cancer isdeveloped through many types of factors including: the smoking factor, the genetic and diseases factors, and the environmental factor.