The following resume is designed to show the methods found especially useful in studying the occurrence of typhoid fever. It is based largely on the writer's experience in the Department of Health in investigating and tracing, during the past seven years, the cause of typhoid fever outbreaks in New York City. It may at once be stated that in a large city like New York, it is practically impossible to trace the source of infection in a large number of unconnected cases. As soon, however, as a series of connected cases occurs the possibility of discovering such source becomes usually only a matter of methods and proper facilities. The first point of importance, therefore, is the recognition of such connected cases. How is this to be accomplished?