This chapter presents the information obtained from observations on man. It considers the evidence—relating to carcinogenic factors that occur generally, or locally, are dependent on the behaviour of the individual, are introduced by medical investigation or treatment, or are related to the occurrence of other diseases. Of the many local factors that could be a cause of cancer the most interesting and potentially the most important is pollution of the atmosphere with chimney smoke and the exhaust fumes of motor vehicles. Perhaps the most useful piece of evidence is that derived from a study of the mortality of gasworkers—men who are exposed to large quantities of tar fumes similar in character to the tar which urban dwellers inspire in town air. The one piece of contrary evidence is that cases have been recognized—albeit very rarely—in children in South Africa, the United States, and Britain who have never visited these tropical areas.