The clinical categorization of a bacterial strain for an antibiotic (SIR system) is based on the in vitro determination of minimum inhibitory concentration and confrontation with breakpoints proposed by national or international committees. The antibiotic spectrum presented in the summary of product characteristics and established by national or European agencies aims at classifying each bacterial species according to their general susceptibility level for the antimicrobial agent. Clinical categorization and antibiotic spectrum are closely related to the definition of breakpoints, established on pharmacokinetics/pharmacodynamics parameters, distribution of minimum inhibitory concentrations for relevant bacteria, acquired resistance rates and if possible, correlation with clinical outcome. This article underlines the need for standardization of susceptibility testing methods, harmonization of breakpoints between European national guidelines, and clinical trial data assessing the correlation between the minimum inhibitory concentration and therapeutic success or failure.