Antibiotic evolution is closely paralleled by the evolution of bacterial resistance. Prior to wide usage of penicillin G, resistance to beta-lactam antibiotics as a consequence of beta-lactamase production had been recognized, and has been an increasing clinical problem ever since. Discovery of antibiotics other than beta-lactams, such as macrolides, tetracyclines and aminoglycosides, has also resulted in the eventual selection of bacteria resistant to these agents. Synthesis of novel beta-lactam derivatives from 6-APA, such as methicillin and isoxazolyl penicillins, resistant to staphylococcal beta-lactamase, overcame the clinical problem of penicillin-resistant S. aureus. Likewise, the isolation of cephamycins and monobactams, and further exploitation of the cephalosporin nucleus, led to the development of derivatives which display a high degree of stability to a wide range of gram-positive and gram-negative bacterial beta-lactamases, thus rendering organisms producing these enzymes susceptible to these agents. Analogous modification of the penicillin nucleus, to give 6 alpha-substituted penicillins, also resulted in derivatives with exceptional stability to beta-lactamases. An alternative approach to the problem of beta-lactamase was the isolation or synthesis of substances able to inhibit the activity of enzymes, thus protecting the unstable beta-lactams from inactivation by beta-lactamase. In this way the activity of beta-lactamase-labile agents was effectively restored against a wide range of beta-lactamase-producing bacterial pathogens. The wide diversity of new antibacterial agents, together with an increasing knowledge and understanding of mechanisms of resistance, indicates that further advances against resistant bacterial pathogens is ensured.