In the latter half of twentieth century, there have been vast improvements in technology for cultivation of button mushroom (Agaricus bisporus). The technology alone, however, can play a role only up to a point and for better and sustainable manifestation of technology, it was important to have proper genotypes (strains). In the twentieth century, the world mushroom production was very low, and button mushroom was the major contributor. Many other edible species of mushrooms have been brought under cultivation in the last few decades. At present, shiitake is the prime contributor to world mushroom production. Gains by technology are a function of genetic improvement. Genetic improvement in button mushroom initially was restricted to selection from available germplasm or use of random methods like multispore cultures and single spore selection. The life cycle of button mushroom was fully understood only in early 1970s, and the first button mushroom hybrid (U1) was developed and commercialized in 1981. Since then, a number of strains have been developed in the Netherlands, the USA, India, and various other countries. Genome of the both the homokaryons of first button mushroom hybrid Horst U1 has been sequenced. Work has been done on WRKY factors, RFLPs, RAPDs, ISSR, MAT locus, etc. These DNA markers for A. bisporus have helped in the identification of homokaryons and varieties, use of approaches like marker aided selection (MAS), and more in-depth study of the genetics. The carbon footprint of mushroom cultivation is low as compared to many other vegetables and animal products. Genetic improvement for producing more from less can further reduce environmental impact and contribute to sustainability. World mushroom production vis-à-vis genetic progress for sustainable growing of button mushroom is discussed in this chapter.