Asthma is a common and costly condition in the United States; 7.5% of the population reports having asthma. Asthma prevalence and mortality rates have been increasing in recent decades; purely biological mechanisms are insufficient to explain asthma onset, exacerbation, or its rising prevalence. This chapter reviews the application of psychological theories to the understanding of asthma; the effects of stress and mood states on asthma; and the prevalence and effects of comorbid psychiatric disorders among persons with asthma. We also provide an overview of medical treatments for asthma, and discuss challenges related to adherence with those treatments. Behavioral and other psychological interventions are reviewed that directly or indirectly affect asthma. Suggestions for future research needs and directions conclude the chapter.