The fluoroquinolones are a series of synthetic antibacterial agents that are undergoing extensive investigation for both human and veterinary use in the treatment of a variety of bacterial infections. These agents work through the inhibition of DNA gyrase, interfering with the supercoiling of bacterial chromosomal material. As a result, these agents are rapidly bactericidal primarily against gram-negative bacteria, mycoplasma, and some grampositive bacteria, with most having little to no activity against group D streptococci and obligate anaerobic bacteria. Resistance develops slowly and is almost always chromosomal and not plasmid-mediated. However, development of resistance to the fluoroquinolones and transfer of that resistance among animal and human pathogens have become a hotly debated issue among microbiologists. The fluoroquinolones are a current antimicrobial class whose use in veterinary medicine is being scrutinized. From a pharmacokinetic perspective, these agents are variably but well absorbed from the gastrointestinal tract and almost completely absorbed from parenteral injection sites, and they are well distributed to various tissues in the body. The fluoroquinolones are metabolized and renally excreted, with many of them having approximately equal excretion by the hepatic and the renal excretory systems. The primary toxicity observed at therapeutic doses involves the gastrointestinal system and phototoxicity, although at higher doses central nervous system toxicity and ocular cataracts are observed. Administration to immature animals may result in erosive arthropathies at weight-bearing joints, and administration of high doses to pregnant animals results in maternotoxicity and occasionally embryonic death. The fluoroquinolones are approved for indications such as urinary tract infections and soft tissue infections in dogs and cats and colibacillosis in poultry. Approval for bovine respiratory disease in the United States is being sought. Other indications for which the fluoroquinolones have been used in animal health include deep-seated infections, prostatitis, and other bacterial infections resistant to standard antimicrobial therapy.