Beta-lactamase production is a common mechanism of resistance, particularly among Gram-negative organisms. Carbapenemases are distinct among the beta-lactams; in addition to being able to hydrolyse most penicillin and cephalosporins, they are also able to hydrolyse the carbapenems. There are two biochemically distinct classes of carbapenemases: the serine beta-lactamases and the metallo-beta-lactamases (MBLs). The fundamental difference between these two classes resides in their hydrolytic mechanisms and rate of hydrolysis. The serine beta-lactamases rely on serine residues within the active site for hydrolytic activity, whereas MBLs rely on zinc ions. Because of these mechanistic differences, these enzymes are inhibited differently; serine beta-lactamases can be inhibited by beta-lactamase inhibitors (such as clavulanic acid), whereas MBLs are inhibited by agents that can chelate divalent cations (such as EDTA). Several MBLs have been described in the literature. The imipenemase (IMP) variety of MBLs was among the earlier beta-lactamases in the class to be identified in the 1980s in Japan. Since their discovery, several IMP variants have been described. Verona integron-encoded metallo-β-lactamase (VIM)-type enzymes are another type of MBL with several described variants, and SPM-type enzymes are another example of MBLs that have been discovered. The New Delhi metallo-beta-lactamase (NDM) is the most recently discovered, globally distributed enzyme.2 This review will focus on the impact of the NDM enzyme on beta-lactam antibiotics.