Attempts to rank causative agents of pneumonia in adults have placed Streptococcus pneumoniae at the top of the list. Until 10 years ago, virtually all pneumococci isolated in the United States were susceptible to less than or equal to 0.02 micrograms/mL penicillin; however, the incidence of disease caused by moderately penicillin-resistant strains has increased in the United States in the past decade. This article reviews the role of S. pneumoniae in causing pneumonia, the best methods to make the diagnosis, and the appropriate means of treatment.