The chapter introduces the concepts of genotype, environment, phenotype, and genotype-by-environment interaction (GEI) in crop breeding. It discusses the implications and causes of GEI, emphasizing its importance in understanding crop performance and environmental factors. The chapter highlights how GEI can influence the heritability of traits, the reliability of phenotype as a predictor of genotype, and the challenges it poses in breeding programs. It also explores the different types of GEI, such as noncrossover and crossover interactions, and their visual representation through biplots. The chapter provides examples to illustrate how even highly heritable qualitative traits can be influenced by the environment, and it discusses the norm of reaction or plasticity concept. Finally, it outlines the negative impacts of GEI on breeding programs, including reduced genetic gain, increased field evaluation costs, and the need for multi-environment trials to identify superior genotypes.The chapter introduces the concepts of genotype, environment, phenotype, and genotype-by-environment interaction (GEI) in crop breeding. It discusses the implications and causes of GEI, emphasizing its importance in understanding crop performance and environmental factors. The chapter highlights how GEI can influence the heritability of traits, the reliability of phenotype as a predictor of genotype, and the challenges it poses in breeding programs. It also explores the different types of GEI, such as noncrossover and crossover interactions, and their visual representation through biplots. The chapter provides examples to illustrate how even highly heritable qualitative traits can be influenced by the environment, and it discusses the norm of reaction or plasticity concept. Finally, it outlines the negative impacts of GEI on breeding programs, including reduced genetic gain, increased field evaluation costs, and the need for multi-environment trials to identify superior genotypes.