In the present stage of development where tuberculosis mortality has lost its statistical significance because of effective chemotherapy, it is generally recognized that the most reliable measure of the extent of the tuberculosis problem in a population is the "annual tuberculosis infection rate" or incidence of infection. In countries where infection with the bovine type of tubercle bacilli no longer exists, as is the case in the majority of low prevalence countries, the annual infection rate expresses that proportion of the population under study which will be primarily infected, or reinfected with tubercle bacilli from a human source in the course of one year. The annual tuberculosis infection rate is also the best measure for following the trend of the tuberculosis problem in a given population and for evaluating the total effects of organized efforts to control tuberculosis.