The failure of patients to adhere to physician-prescribed regimens, either pharmacologic or behavioral, has been well documented in medical literature. Poor adherence to asthma medication regimens has been repeatedly demonstrated in both children and adults, with rates of nonadherence commonly reported from 30 to 70%. Medication regimens for asthma care are particularly vulnerable to adherence problems because of their duration, the use of multiple medications, and the periods of symptom remission. The clinical effects of this nonadherence by asthmatic patients can include treatment failure, unnecessary and dangerous intensification of therapy, and costly diagnostic procedures, complications, and hospitalizations. Although the measurement of adherence is an important component of both medical and behavioral interventions to control asthma, relatively little research has directly addressed the reliability and validity of the measures most widely used to assess asthma medication compliance. This review will discuss methods and issues in the measurement of adherence in general, and where available, measures that have been specifically used in evaluating adherence to asthma medication. Common measures used to assess compliance with asthma medications include direct measures, which confirm the use of medication by assaying it in blood, urine, or saliva, or which confirm the ability to use a medication, such as observed skill in using a metered dose inhaler. Indirect measures infer use with varying degrees of reliability, by use of clinical judgment, self-report/asthma diaries, medication measurement, and electronic medication monitors. The uses and limitations of these measures will be discussed.