The introduction of hybrid maize varieties in the 1930s in the USA was instrumental in driving a rapid and continuing increase in maize yields. Maize breeding for hybrid development depends on two main approaches, pedigree breeding and population improvement. Access to diverse and appropriate germplasm is essential as the foundation for the populations is to be improved. Practically speaking, maize is monoecious (separate male and female flowers on the same plant); this makes controlled hybridization for breeding fairly simple, and also allows for the production of hybrid maize seed. Objectives of maize breeding include yield, resistance to biotic (e.g., disease) and abiotic (e.g., drought) stresses, and grain quality.