Reasons for the use of antibacterial combinations are synergistic effects and delayed development of bacterial resistance. On the basis of numerous experiments in vitro the opinion is supported that a clinically useful synergism depends on the addition of an aminoglycoside, the second compound being either one of the penicillins or cephalosporins, or a polymyxin, or a bacteriostatic agent like tetracycline, macrolide, and lincomycine. The decrease of MBC and MIC values varies according to the species and strains of bacteria and in dependence on the components of antibiotic combinations. Combination therapy with antibacterial synergism is indicated in life-threatening infections and in order to suppress bacterial persistences. Combinations with antagonistic effects should be avoided. The use of multiple antibiotics for broader spectrum only is not supported.