Antimicrobial agents have been in use for therapeutic purposes for over a century, with most of the development occurring in the latter half of the twentieth century. Penicillin was the first of the naturally occurring antimicrobials to be used in medicine and its structure includes a beta-lactam ring. Further compounds, such as the cephalosporins, were discovered and these also included beta-lactam structures. Subsequently bacteria which were resistant to these compounds were found, and their resistance was due to their production of enzymes, beta-lactamases, which hydrolysed the beta-lactam ring. Synthetic derivatives of the beta-lactam antimicrobials were developed to render them recalcitrant to beta-lactamases but enzymes with a broader substrate range evolved, and were categorised as extended substrate beta-lactamases (ESBL). Since the antimicrobials had a significant role to play in human medicine the emergence of ESBL caused significant concerns. Further, similar antimicrobials were used by veterinarians, raising the prospect that bacteria in the commensal flora of livestock could acquire ESBL resistance properties and exchange them via genetic exchange. Thus, pathogenic bacteria present in livestock could become resistant to antimicrobials with adverse consequences should zoonotic infections occur. In this review we consider the emergence of ESBL, the problems involved in detecting and reporting such properties, and consider the consequences for consumers of potentially contaminated food products