BCG vaccination has been given to infants and children so extensively and repeatedly for more than 55 years in Japan that it is very difficult, or often impossible to diagnose tuberculosis infection by tuberculin testing. On the other hand, as the treatment of latent tuberculosis infection has become more and more important recently, diagnosis of infection is becoming more and more important at the occasions of contact survey. However, understanding of the health care workers about transmission of tuberculosis is incomplete frequently at present. This is the reason why the author has written this review. The author has described on the history of the progress of droplet nuclei infection theory, infectivity of tuberculosis by bacteriological status of the patients, importance of cough, susceptibility of the host, and environmental factor concerning transmission of tubercle bacilli in this issue.