Patient adherence with prescribed inhaled therapy is related to morbidity and mortality. The terms "compliance" and "adherence" are used in the literature to describe agreement between prescribed medication and patient practice, with "adherence" implying active patient participation. Patient adherence with inhaled medication can be perfect, good, adequate, poor, or nonexistent, although criteria for such levels are not standardized and may vary from one study to another. Generally, nonadherence can be classified into unintentional (not understood) or intentional (understood but not followed). Failing to understand correct use of an inhaler exemplifies unintentional nonadherence, while refusing to take medication for fear of adverse effects constitutes intentional nonadherence. There are various measures of adherence, including biochemical monitoring of subjects, electronic or mechanical device monitors, direct observation of patients, medical/pharmacy records, counting remaining doses, clinician judgment, and patient self-report or diaries. The methods cited are in order of more to less objective, although even electronic monitoring can be prone to patient deception. Adherence is notoriously higher when determined by patient self-report, compared to electronic monitors. A general lack of adherence with inhaled medications has been documented in studies, and adherence declines over time, even with return clinic visits. Lack of correct aerosol-device use is a particular type of nonadherence, and clinician knowledge of correct use has been shown to be imperfect. Other factors related to patient adherence include the complexity of the inhalation regimen (dosing frequency, number of drugs), route of administration (oral vs inhaled), type of inhaled agent (corticosteroid adherence is worse than with short-acting beta2 agonists), patient awareness of monitoring, as well as a variety of patient beliefs and sociocultural and psychological factors. Good communication skills among clinicians and patient education about inhaled medications are central to improving adherence.