Given the close links between retention and outcome in substance abuse treatment, it is important to recognize that treatments are successful to the degree they retain patients. This chapter described some practical strategies for improving retention in clinical trials of treatment for substance abuse. To summarize: 1. Retention can be conceived as an important treatment outcome that reflects good fit between patient, therapist, treatment, and setting. Procedures and practices that improve the quality of treatment are likely to also improve retention. 2. Attending to the problem of retention may help solve the problem. While trials are ongoing, investigators should monitor retention closely, attending to and addressing variations in retention that might be associated with setting, seasonal variations, therapist factors, and research procedure factors. 3. More data are needed on effective methods of enhancing retention in different treatment settings. It should be noted that the strategies presented here reflect common sense and are for the most part drawn from experience with several clinical trials. Few of them have been evaluated empirically. However, more data on effective retention strategies are likely to have broad clinical and research utility. For example, it would be possible to design studies that evaluate an adaptation of Higgins' voucher system (this volume) to specifically reinforce retention in treatments that have higher rates of attrition, different methods of rewarding clinicians with higher rates of retention, and the effect on retention of adding babysitting services, to mention but some areas where further research would be illuminating.