The principal characteristics of maize and its two relatives, teosinte and Tripsacum are described. The manifestations of the introgression of teosinte into maize determined from experimental cultures can be recognized in prehistoric maize from Guatemala, Mexico, and the southwestern United States. This introgression, which began at least 2,000 years ago, is still proceeding in Mexico where maize and teosinte commonly grow in the same fields and hybridize frequently. Evidence is presented of the direct introgression of Tripsacum in several races of maize of South America. The introgression of teosinte and Tripsacum has contributed to the evolution of maize through gene recombination, mutagenic effects, and various types of heterotic phenomena. The role of introgression in the improvement of maize is discussed.