When planning user studies, practitioners should estimate the sample sizes required to meet their goals. This is most critical when the cost of adding to a sample is high. When estimating sample sizes, the appropriate method depends on the type of study. This chapter covers methods for user studies (such as summative usability studies) that use measurements that are continuous (such as time on task), multipoint scale (such as usability questionnaires), and discrete (such as successful task completions). The sample size estimation methods are derived from the formulas for z-tests, t-tests, and binomial tests.