Many new bacteria appear each year, and researchers are challenged to develop new antibiotics—chemicals that inhibit specific bacteria—to deal with each new threat. Antibiotics are made in three ways: by living microorganisms, by synthetic manufacture, and, in some cases, through genetic engineering. Antibiotics may either be bacteriostatic (preventing the growth of bacteria) or bactericidal (killing bacteria directly), although several antibiotics are both bactericidal and bacteriostatic, depending on the concentration of the particular drug. This chapter discusses the major classes of antibiotics: aminoglycosides, carbapenems, cephalosporins, fluoroquinolones, penicillins and penicillinase-resistant drugs, sulfonamides, tetracyclines, and the disease-specific antimycobacterials, including the antitubercular and leprostatic drugs. Antibiotics that do not fit into the large antibiotic classes include ketolides, lincosamides, macrolides, and monobactams. show sites of cellular action of these classes of antibiotics.