beta-Lactam antibiotics are the most frequently prescribed antibiotics worldwide. Therefore, it is not surprising that resistance to this very important class of agents poses an increasingly complex and perplexing problem for physicians. Among the variety of mechanisms that can provide resistance to beta-lactam antibiotics in gram-negative bacilli, the production of beta-lactamase is by far the single most important factor. With the introduction of newer beta-lactam agents observed changes in beta-lactamases include the increased prevalence of older enzymes, the appearance of new enzymes, and alteration in the level of expression of the enzymes. These changes have been responsible for resistance to newer cephalosporins, monobactams, carbapenems, and beta-lactamase inhibitor/beta-lactam drug combinations. Resistance to beta-lactam antibiotics has also emerged through alterations in the targets of the drugs, the penicillin-binding proteins, and through alterations in outer membrane permeability of the organisms to the drugs. With some beta-lactam agents, multiple mechanisms must be acquired before clinically relevant levels of resistance are attained. This is especially true for carbapenems and fourth generation cephalosporins. Nevertheless, resistance to beta-lactam antibiotics is on the rise among clinical isolates of gram-negative bacilli, and only through more judicious use of these agents can their usefulness for treatment and prevention of infections be preserved.