Smoking is the leading risk factor for several serious diseases and causes an enormous economic burden for the individual and society. Approximately six million deaths in the world annually are attributable to smoking. Thus, it is urgent to enhance the awareness of the harm caused by smoking and to develop additional effective ways to achieve smoking cessation. In the past decades, a great number of epidemiological studies have been performed to reveal the patterns of smoking and its associated diseases. In this chapter, we briefly introduce smoking prevalence worldwide and overview the investigation of smoking-associated diseases, including cancers and psychiatric disorders.