The antibiotics used in the chemotherapy of microbial infections are considered with respect to their antimicrobial spectra and their mechanisms of action, respectively. The sites of action of the different groups of antibiotics interfering with the same cellular processes are discussed. According to their primary actions on sensible cells the antimicrobial antibiotics are divided into four groups: 1. Inhibitors of the biosynthesis of the bacterial cell wall 2. Inhibitors of the bacterial protein synthesis 3. Inhibitors of the nucleic acid metabolism 4. Membrane-active antibiotics.