The European Committee on Antimicrobial Susceptibility Testing (EUCAST) has redefined the antimicrobial susceptibility testing categories S, I and R. Traditionally these are defined as S = susceptible, I = intermediate and R = resistant. However, these definitions do not adequately take into account the pharmacokinetic (Pk) and pharmacodynamic (Pd) aspects of antimicrobial usage. Dose optimization of antimicrobials is currently considered of paramount importance in successful treatment of infectious diseases, and this is achieved through targeting the optimal Pk-Pd parameter associated with each antimicrobial agent.