Chemotherapy is defined as use of chemicals in infectious diseases to destroy microorganisms without damaging the host tissue. Substances produced by microorganisms, which inhibit the growth or destroy other microorganism are termed as antibiotics. Antimicrobial agents are classified on the basis of mechanism of action, chemical structure, type of action, and spectrum which are discussed briefly in this chapter. Site, concentration, host defence, and sensitivity are the factors influencing successful chemotherapy. Antimicrobial resistance, selection of appropriate advanced maternal age, antimicrobial agents combinations, chemoprophylaxis, superinfection (suprainfection), and probiotics are the other topics which are discussed broadly in this chapter.