The $\beta$-lactamases are widely distributed in both Gram-positive and Gram-negative bacteria. They all inactivate penicillins and cephalosporins by opening the $\beta$-lactam ring. Many varieties of the enzyme can be distinguished on the basis of their catalytic and molecular properties, but only amino acid sequence determination gives information upon which a molecular phylogeny can be based. The present evidence suggests that the $\beta$-lactamases have a polyphyletic origin. All the $\beta$-lactamases of currently known amino acid sequence belong to one homology group, here called class A enzymes. Class B consists of the mechanistically distinct Bacillus cereus $\beta$-lactamase II, which preliminary partial sequence analysis suggests to be structurally unrelated to the class A enzymes. It is predicted that sequence analysis will show that further classes will need to be created to account for particular $\beta$-lactamases of distinctive molecular and mechanistic properties.