For 2 decades fluoroquinolones have been found to be generally well-tolerated and safe. Adverse events may be inherent to the class or influenced by structural modifications. The commonest adverse events are gastrointestinal tract (GI) and central nervous system (CNS) reactions; nephrotoxicity and tendinitis are infrequent, but agents differ greatly in phototoxic potential. Fluoroquinolones are safe in elderly, human immunodeficiency virus-infected, and neutropenic patients, but because of possible effects on articular cartilage, they are not currently recommended for children or pregnant women. Four new agents have recently been licensed. Levofloxacin causes few GI or CNS adverse events and is minimally phototoxic. Sparfloxacin infrequently causes GI or CNS effects but is associated with relatively high rates of phototoxicity and prolongation of the electrocardiographic QTc interval (Q-T interval, corrected for heart rate). Grepafloxacin causes relatively high rates of GI effects, taste perversion, and QTc interval prolongation, but it is minimally phototoxic. Trovafloxacin is associated with a moderate rate of GI effects and a relatively high incidence of dizziness but has low phototoxic potential.