Antimicrobials have become indispensable in decreasing morbidity and mortality associated with a host of infectious diseases and, since their introduction into veterinary medicine, animal health and productivity has improved significantly. Antimicrobial resistance mechanisms have been reported for all known antibiotics currently available for clinical use in human and veterinary medicine. On a long-term evolutionary scale, the epidemiology of antimicrobial resistance should be regarded as dominated by the stochastic or chaotic movement of resistance genes within a gigantic bacterial genetic pool. Today, many clinically important bacteria are characterized by multiple antibiotic resistance phenotypes, the legacy of past decades of antimicrobial use and misuse. Resistance is an inevitable biological phenomenon: the challenge is to prevent it from continuing to be a persistent and serious obstacle to modern medicine.