Streptococcus pneumoniae is a fastidious obligate parasite, colonizing man and, occasionally, other mammals. Its nutritional requirements are complex, resembling to a large degree those of its hosts (Tomasz, 1964). Its ecological niche is the nasopharynx; and, when localized to that area, it enjoys a commensal relationship with the subject it inhabits. Only after extending to other areas of the respiratory tract or after penetration of the nasopharyngeal mucosa to reach the systemic circulation via the cervical lymphatics does it give rise to disease which may ultimately work to the detriment of both host and parasite. It is the purpose of this brief review to examine some of the factors which influence the relationships between man and the pneumococcus.