This chapter discusses genetics and plant breeding. Crops are demanded for many purposes and must cope with stresses from weather, pests, and various ecosystem services; plants are bred by farmers, scientists, and seed companies to fill those various needs. The phenotypes of the plants we know today are the result of scientists crossbreeding plants and seeing how their genotypes interact with the environment. Each gene has a small effect; genetic diversity is the raw material underlying plant breeding, which relies on a process of crossing and recombining different sets of genes and selecting them. The factors that determine the speed with which the progresses advance include the strength of selection, accuracy of selection, genetic diversity that’s being selected upon, and how fast the genetic cycles occur. Phenotype innovation can be done for macroscopic traits like yield, or microscopic traits like root hairs. There are three kinds of breeding for intensification; energy intensification or input intensification, sustainable intensification, and agroecological intensification. The Green Revolution was an era of energy intensification, to make plants that could make use of fertilizer and irrigation to produce higher yields and make iterative famines a thing of the past. Sustainable intensification looks not just for input responsiveness, but for input use efficiency and tolerance of stresses like flocking, inadequate soil nutrients, etc. Case study of Vietnamese farmers development blast-resistant varieties of plant, among others. Agroecological intensification means substituting inputs using agroecological principles. Breeding can be used to improve the performance of a crop or to improve the chemical ecology of a plant, etc. Case study of Bioversity International, an organization that works with local NGOs to engage farmers in plant breeding activities.