The current issue is devoted broadly to research on treatment adherence and chronic illness self-management behavior. As the prevalence of chronic illness increases, the pervasive problem of treatment nonadherence is increasingly viewed as having a major impact on treatment outcomes, public health and healthcare costs, making this issue particularly timely. Sixteen articles spanning an array of topics are presented; articles include empirical studies, statistical simulations, systematic reviews, and theoretical commentaries. Studies conducted with diverse patient populations (e.g., chronic headache, diabetes, end-stage renal disease, HIV, hypertension, severe obesity), samples (e.g., adolescents, ethnic/racial minorities, low-income adults, parents, spousal dyads), and designs (e.g., cross-sectional, longitudinal assessment, randomized controlled trial), are represented. This issue highlights psychosocial factors associated with nonadherence, promising interventions to promote adherence, and state-of-the art methods for the study of illness self-management. We hope these articles engender even more high quality, methodologically rigorous research in this important subfield of behavioral medicine.