This chapter traces the evolution of Ashbya gossypii from a cotton pathogen into a versatile microbial platform for industrial biotechnology. Initially identified in the 1920s as the causative agent of stigmatomycosis, A. gossypii was later shown to rely on insect vectors for plant colonization, revealing a symbiotic lifestyle. Its natural capacity to overproduce riboflavin (vitamin B2) attracted early industrial interest and laid the foundation for decades of applied research. Despite early taxonomic uncertainties, molecular and genomic studies uncovered a close evolutionary relationship with Saccharomyces cerevisiae. This, combined with a compact genome and efficient homologous recombination, established A. gossypii as a genetically tractable model organism. Advances in transformation and gene-targeting tools during the 1990s enabled precise metabolic engineering, cementing its leading role in sustainable industrial riboflavin production. From the 2010s onward, continued advances in metabolic engineering and synthetic biology broadened its industrial applications beyond riboflavin production. Since then, A. gossypii has been engineered to produce a growing portfolio of valuable bioproducts, including nucleosides, orotic acid, folates, biolipids, lactones, terpenes, gangliosides, and recombinant proteins. This remarkable journey from plant pathogen to industrial microbial cell factory highlights the central role of A. gossypii in advancing sustainable “white biotechnology”.