AmpC β-lactamases have been a target of study since the late 1970s. Most of these enzymes are cephalosporinases but are capable of hydrolysing all β-lactams to some extent.1,2 Researchers have examined characteristics of both inducible and non-inducible AmpC β-lactamases such as physical properties, hydrolytic activity, the molecular mechanisms involved in chromosomal expression, and comparative studies between genera on the induction potential of the enzyme.1,3 In the late 1980s, these inducible chromosomal genes were detected on plasmids (most without induction capabilities) and were transferred to organisms, which typically do not express these types of β-lactamase such as Klebsiella spp., Escherichia coli, or Salmonella spp. The plasmid-encoded or ‘imported’ ampC β-lactamase complicates the job of clinical microbiologists working in hospital laboratories. No longer can a Gram-negative organism be considered a potential AmpC-producing organism based on identification. In addition, many clinical microbiologists are unaware of plasmid-encoded AmpC β-lactamases because phenotypic detection is difficult at best and these β-lactamases can be misidentified as extended spectrum β-lactamases (ESBLs). This article serves to point out new developments and/or gaps in the basic knowledge of our understanding of AmpC β-lactamases.