Asthma is a chronic, inflammatory disease of the airways that has dramatically increased in incidence over the past 15 years in both developed and developing countries. The global burden of asthma is considerable. Its effects include reduced quality of life, lost productivity, missed school days, increased health care costs, the risk of hospitalization and even death (1).

Although effective treatments that have been shown to dramatically reduce asthma morbidity are available, they are effective only when properly used by patients. Because human behaviour is the necessary interface between good therapies and therapeutic effectiveness, both clinical researchers and clinicians should understand the factors associated with patient adherence. This chapter discusses adherence issues in asthma, with a particular focus on adherence to preventive therapy, such as inhaled corticosteroids (ICSs).The prevalence of nonadherence to preventive therapy and patient factors associated with nonadherence are reviewed. Finally, we suggest some directions for future field research.