For many decades, the term “polymorphism” has been used to describe genetic variants in population genetic studies. A number of different kinds of polymorphisms have been identified during this time, both protein and DNA forms, largely driven by advances in methods for assessing genetic sequence variation. This entry briefly summarizes the history and results of efforts to evaluate polymorphisms in human and primate populations. Our knowledge of the sequence composition and functional features of the human genome continue to grow with new approaches to understanding its sources of variation and their phenotypic consequences.