As indicated in Chapter 1 , randomized controlled trials (RCTs) are considered the gold standard for establishing causality between exposure and intervention. To achieve its objectives, that it is to report an unbiased treatment effect, complete data should be available during and at the conclusion of the study. Missing or incomplete data are a serious problem in many fields of research and are ubiquitous in medical research, even RCTs. What we mean by missing data points is unavailable data, whether for the response or one or more covariate values (item missing) or an entire questionnaire. The issue of missing data is in particular very frequent in longitudinal or repeated-measures studies. Often, we use the term missing to include both attrition and noncompliance.