Theophylline is an important antiasthmatic medication which has bronchodilator properties. With increased understanding of the relationships of serum theophylline concentration and effect, both adverse and beneficial, oral dosage forms were developed to provide consistent serum theophylline concentrations with the benefit of convenient dosage intervals for long term use. Since factors such as concurrent disease states, drug interactions and age have a profound effect on theophylline disposition, relatively sophisticated dosage guidelines have evolved. Theophylline is in fact a model drug for the application of pharmacokinetic principles to the individualization of a treatment regimen. The purpose of this discussion is to review the relationship of serum theophylline concentration and pharmacodynamic effect and the special properties of oral sustained release theophylline formulations, and to provide a practical approach to prescribing theophylline. Guidelines are provided on the use of serum theophylline concentrations to individualize the theophylline dose, with an analysis of available techniques to monitor theophylline.