The aminoglycoside antibiotics are indispensible therapeutic agents; however, a high incidence of toxic effects mandates that they be used with caution. In many patients, careful monitoring of serum concentrations is required. Because of their chemical characteristics, aminoglycosides are given parenterally except in certain well-defined clinical situations such as bowel preparation prior to surgery. The drugs act by interfering with microbial protein synthesis and are uniformly bactericidal. Plasmid-mediated resistance remains a problem, particularly in the hospital environment. Aminoglycosides find their most frequent use in infections caused by aerobic gram-negative bacilli. The pharmacokinetics have been well characterized, and an appropriate dosage regimen can be selected to achieve peak bactericidal levels while maintaining trough concentrations low enough to minimize toxicity. Monitoring of serum aminoglycoside concentrations is essential in many types of patients, such as those having compromised renal function, those receiving a maximum dose of the drug, the obese, the elderly, and patients with cystic fibrosis. Close cooperation between the clinical laboratory and all groups involved in the patient's care is necessary to make maximum clinical use of drug monitoring information.