Genotype-by-Environment Interaction (GEI) is a prevalent issue among crop farmers, plant breeders, geneticists, and production agronomists. This book brings together contributions from expert plant breeders and quantitative geneticists to better understand the relationship between crop performance and environment. This information can reduce the cost of extensive genotype evaluation by eliminating unnecessary testing sites and by fine-tuning breeding programs. Molecular aspects of GEI are discussed for the first time and key bibliographical references on GEI are included in an appendix.