The discovery of antibiotic substances was probable the greatest achievement of medicine, because they allow treating many infectious diseases or syndromes and curing them. Only the substances that killed bacteria are called antibiotics, and those against other microorganisms are called antimicrobials. It is very important to understand the role and functions of antibiotics, how they inhibit or kill bacterial cells and their adverse events. Once the antibiotics were widely used, the resistant bacterial strains appeared. Many bacteria become resistant to such substances and the mechanisms of resistance are not complete understood yet in some cases. Therefore it is necessary to perform the antibiogram and to use the appropriate treatment with efficient antibiotics. Also the scientists search for new substances with antibacterial activity (new antibiotics, synthetic substances, plant extracts) to avoid the induction of antibiotic resistance and to have success in therapy of bacterial infections.