Early works leading to the detection of the pneumococcus and eventually to the appreciation that isolates differed in agglutination and that antisera differed in their capacity to protect against pneumococcal infection in the mouse protection test are reviewed. Studies by researchers from Europe, South Africa, and the United States over nearly five decades led to the introduction of serum therapy. Rapid typing methods thus became very important, and type-specific serum therapy generated a dramatic decrease in the number of deaths from pneumococcal pneumonia. Just before the introduction of sulfonamides and, shortly thereafter, penicillin, the use of horse sera was replaced by the use of rabbit sera for a number of reasons. The present methods of typing comprise the capsular reaction test, latex- and coagglutination, and capillary precipitation, to name the most important; these use a large variety of antisera. Newer methods include the use of DNA probes and DNA sequence-based subtyping.