Although the means whereby pneumococcus damages the hosts it invades remain unknown, some advances in understanding the biology of this organism and the pathogenesis of pneumococcal infection have been made in recent years. Knowledge of the composition of the cell wall (C) polysaccharide of the pneumococcus and its relation to the capsular polysaccharides of this organism is considered and a hypothesis of the evolution of the pneumococcal capsule is entertained. Refinements in understanding the pathogenesis of pneumococcal pneumonia, bacteremia, and otitis media are reviewed, and current knowledge of the epidemiology of pneumococcal infections and of the distribution of pneumococcal types causing such infections is discussed. The history of the development of pneumococcal vaccines is summarized briefly, together with an account of the field trials of contemporary polyvalent vaccines of pneumococcal capsular polysaccharides conducted recently.