This systematic review examines adherence and dropout rates in randomized controlled trials (RCTs) of Internet-based interventions for anxiety and depression. The study found that dropout rates in RCTs were lower (1-50%) compared to open access websites, where dropout is more common. Factors influencing adherence included disease severity, treatment duration, and chronicity. However, few studies formally examined reasons for dropout, and most failed to use appropriate statistical methods for analyzing missing data. Adherence was generally high, ranging from 50-70% for depression sites and 50% for generalized anxiety disorder (GAD). Predictors of adherence included lower baseline depression symptoms, younger age, and poorer knowledge of psychological treatments. Reasons for dropout included time constraints, lack of motivation, technical problems, and perceived lack of treatment effectiveness. The review highlights the importance of developing theoretical models of adherence in Internet intervention research. It also emphasizes the need for better statistical methods to handle missing data, such as multiple imputation and maximum-likelihood methods. The findings suggest that Internet-based interventions may be more effective than open access websites in terms of adherence, but further research is needed to understand the factors influencing adherence and dropout in both settings. The study underscores the importance of comparing adherence rates between open access interventions and traditional health services to improve the effectiveness of Internet-based mental health interventions.This systematic review examines adherence and dropout rates in randomized controlled trials (RCTs) of Internet-based interventions for anxiety and depression. The study found that dropout rates in RCTs were lower (1-50%) compared to open access websites, where dropout is more common. Factors influencing adherence included disease severity, treatment duration, and chronicity. However, few studies formally examined reasons for dropout, and most failed to use appropriate statistical methods for analyzing missing data. Adherence was generally high, ranging from 50-70% for depression sites and 50% for generalized anxiety disorder (GAD). Predictors of adherence included lower baseline depression symptoms, younger age, and poorer knowledge of psychological treatments. Reasons for dropout included time constraints, lack of motivation, technical problems, and perceived lack of treatment effectiveness. The review highlights the importance of developing theoretical models of adherence in Internet intervention research. It also emphasizes the need for better statistical methods to handle missing data, such as multiple imputation and maximum-likelihood methods. The findings suggest that Internet-based interventions may be more effective than open access websites in terms of adherence, but further research is needed to understand the factors influencing adherence and dropout in both settings. The study underscores the importance of comparing adherence rates between open access interventions and traditional health services to improve the effectiveness of Internet-based mental health interventions.