The internal and external validities of a clinical research study are threatened when enrolled participants are lost or drop out before the study is completed. Cognitive impairment, financial stress, housing instability, substance abuse, and transportation access are common factors in attrition in studies with some clinical populations. This chapter outlines the implications of and common reasons for attrition, and provides strategies to enhance retention. Those with complete data often differ significantly from those lost to attrition. Bias occurs when lost data are not random and are related to the outcome(s). Bias may confound associations and distort prevalence estimates. Statistical techniques used to account for missing data assume that those who are missing have results like the observed cases with similar measured characteristics.