Current knowledge of the morphology, biochemistry and function of streptococcus Pneumoniae, the immune response of the host to infection and certain epidemiological aspects of pneumococcus infections is described. Personal experience of acute infections of the lower respiratory ways and meninges is presented and therapeutic problems are discussed. In Italy at least it appears that betalactamine antibiotics are preferable. Brief reference is made to the value and limitations of anti-pneumococcic vaccination.