As betalactams (penicillins, cephalosporins, carbapenems, monobactams, betalactamase inhibitors) are the most widely used antibiotic drugs worldwide, allergy to betalactams is a frequent problem encountered in clinical practice, concerning from 0.7-8% of treated patients. It is now clear that the antigenicity of these drugs is strictly related to the chemical structure of the various molecules and of their constituent parts. In this paper we first describe the chemical structure of betalactams and then try to establish a correlation between this structure and their antigenicity.