The success or failure of antimicrobial therapy in bacterial and fungal infections is predicted ideally by antimicrobial susceptibility testing (AST), in which microorganisms are divided into treatable and non-treatable categories on the basis of MIC breakpoints. In Europe, the categorization was traditionally a clinical one and it was made irrespective of whether or not the organism harboured resistance mechanisms. MIC breakpoints generally divide bacteria into three categories of susceptibility: susceptible, intermediate or indeterminate, or resistant. These terms can be defined as susceptible (S—where the antimicrobial activity is associated with a likelihood of therapeutic success), intermediate or indeterminate (I—where the antimicrobial activity is associated with an indeterminate or uncertain therapeutic effect) and resistant (R—where the antimicrobial activity is associated with a higher than expected likelihood of therapeutic failure). MIC breakpoints are used either directly, as in MIC determination and ‘breakpoint’ susceptibility testing methods in broth or agar, or indirectly when converted into inhibition zone diameters in disc diffusion techniques.