The therapeutic program for persons with asthma includes recommendations for altering the environment and a drug regimen designed to alleviate symptoms, minimize exacerbations, and improve quality of life. Unfortunately, patients can have difficulty adhering to these recommendations, which contributes to treatment failure and increased costs. This paper provides a comprehensive review of the challenge of adherence in adults with asthma, including the costs and benefits, optimal adherence levels, assessment methods commonly used in research and practice, factors believed to predict poor adherence, and tested and untested strategies for improving adherence. Opportunities for further research are discussed throughout the paper.