Some of the major contributions to biology and medicine resulting from studies of the pneumococcus (Streptococcus pneumoniae) and of the diseases caused by it in the century following its initial isolation are reviewed briefly. Among the topics considered are cellular and humoral defenses against pneumococcal infection, the role of pneumococcus in the development of quantitative immunochemistry, the historical position of the pneumococcus in the recognition of bacterial resistance to drugs, and the fundamental role played by this organism in the origins of molecular genetics and molecular biology. Because of the vast store of knowledge of the pneumococcus that has been acquired in the past 100 years, the hope is expressed that this knowledge will continue to grow and to serve as a basis for the extension of understanding both the bacterial cells and of the processes of infection.