As currently defined, antibiotics are chemicals produced by microorganisms and which in low concentrations are capable of inhibiting or killing other microorganisms. In the last few years, this definition has been somewhat broadened by some authors to include materials produced by all living systems (plants, animals, or microorganisms) which inhibit any cell activity. Antibiotics may be wholly produced by fermentation. Nowadays however, they are increasingly produced by semi-synthetic processes, in which a product obtained by fermentation is chemically modified. Some antibiotics, e.g. chloramphenicol, were originally produced by fermentation, but are now more cheaply produced by chemical means. Thousands of antibiotics are known; however, a small proportion of known antibiotics is used clinically. The antitumor antibiotics are heterogeneous in their chemical natures. Some of the best known groups used in clinical practice include anthracyclines, actinomycins, and bleomycins. This chapter explains classification and nomenclature of antibiotics, beta-lactam antibiotics, the search for new antibiotics, the need for new antibiotics, combating resistance and expanding the effectiveness of existing antibiotics, semi-synthetic antibiotics, antitumor antibiotics, mode of action of antitumor antibiotics, search for new antitumor antibiotics and newer methods for the search for antibiotic and antitumor drugs.