Aminoglycosides are widely used antibacterial agents, particularly for serious infections. They have a narrow therapeutic margin of efficacy over toxicity relative to many other agents, a disadvantage which can be obviated to some extent by precise dosing regulated by blood concentration monitoring. The relationships of efficacy and toxicity to concentrations are discussed, as are the practical aspects of clinical indications for monitoring and specimen collection. Dosing by predictive methods is useful but not always of sufficient precision. A large number of methods for assay are now available. With care, microbilogical plate assays can give results of adequate accuracy and selectivity, and with sufficient speed. Immunoassays give more accurate, specific and rapid answers; EMIT, in particular, seems the best method available at the present time. Whichever method is used, control of its accuracy is essential.