The survey sampler may know additional information about the population before sampling, such as the gender and major of every student at a university, the percentage of persons in each county who are age 65 or older, or the dollar amount of every loan made by a bank. Stratified sampling uses this additional information about the population to improve the efficiency of the survey design. This chapter shows how to use R functions to allocate and draw stratified samples from a population with proportional, Neyman, optimal, or custom allocation. In addition, it provides code and output for estimating population means, totals, and percentages from the stratified random sampling examples in Chapter 3 of Sampling: Design and Analysis, Third Edition.