 In this entry, I review classical ecological genetics and then discuss the novel kinds of processes and explanations that accompanied the expansion of the field from single populations to genetically structured metapopulations and from phenotypic to biochemical and DNA sequence polymorphisms. I will show that the central early controversy over the roles of random genetic drift and natural selection in evolution has continued to this day, despite the apparent technological refinements afforded by the availability of biochemical and DNA sequence data. That is, finer scale or more reductionistic genetic data has not yet led to a resolution of the original conceptual issues that lie at the foundation of ecological genetics.