Three species of bacteria (Haemophilus influenzae type b, Neisseria meningitidis, and Streptococcus pneumoniae) cause approximately three-quarters of all cases of acute bacterial meningitis in industrialized and developing countries. Infections due to N. meningitidis, S. pneumoniae, and H. influenzae type b are endemic in most countries; major epidemics of meningococcal disease still occur regularly, especially in sub-Saharan Africa. Such epidemics may be large, involving many thousands of patients, with a mortality that can exceed 10%. Both chemoprophylaxis and immunization are used to prevent meningococcal, pneumococcal, and H. influenzae type b meningitis. Chemoprophylaxis may involve the use of expensive antibiotics, and it can encourage the emergence of drug resistance. Mass immunization with meningococcal polysaccharide vaccine can effectively halt an epidemic of group A or group C meningococcal disease, and immunization protects close contacts. However, polysaccharide vaccines are ineffective in infants, who are very susceptible to bacterial meningitis. New proteinpolysaccharide conjugated vaccines may be more effective in this young population.