As in many countries, the introduction of modern crop varieties and the replacement of local land races and their wild relatives have conducted to the erosion of genetic diversity. Extensive research studies designed to conserve, to characterize and to utilize the genetic resources of maize were conducted in México during the last 25 years. This work describes the collection missions of the closest wild relatives of maize and the results on morphological, physiological, genetic and molecular characterization of Zea species. Currently, the collection of teosinte has around 1000 accessions, maintained, mainly as seeds in three germplasm banks in México. The discovery of morphologically and ecologically distinct new populations of teosinte from México motivated a comparative study of populations from all known species; evidence from multiple sources supports the taxonomic separation of three new species from the rest of taxa within the genus Zea. Diversity studies have considered around 800 maize and teosinte accessions. Characterization included: 1) Geographic distribution of cross-incompatibility factors in wild and weedy species and cultivated maize; 2) Seed dormancy in Mexican teosinte; 3) Isozyme and microsatellite markers for genetic diversity estimation.