The fifth of a series of seven H3ABioNet online lectures for Genome Wide Association Studies (GWAS) will introduce the concept of population structure/stratification (PS) and suggest why it is critical to consider PS in a GWAS. The lecture will begin by familiarizing the types of PS that are encountered and explain the factors that could cause them to appear in a GWAS dataset. It will then cover the methods that are commonly used to identify PS in a dataset and finally discuss the available computational approaches to correct for PS in a GWAS.