In Chapter 12 sample size calculations for longitudinal studies are discussed. It is shown that a sample size calculation for longitudinal studies is based on the standard sample size calculation formula with an additional multiplication factor which includes the number of follow-up measurements and the estimated correlation between the repeated measurements. Sample size calculations for both a continuous and a dichotomous outcome variable are illustrated with an example. Besides the practical part of the sample size calculation, the chapter also includes a discussion regarding the sense or nonsense of sample size calculations.