Walnuts as a crop has expanded considerably in production and value over the last 30 years. There has been a shift in dominant production area from California to China. While the United States is still the world’s largest exporter of walnuts China now dominates world production. This chapter provides detailed statistics on the expansion of walnut production, examining key issues around what is now a highly diversified production environment, including blight (Xanthomonas arboricola pv. Juglandis) management and new potential methods for control. The chapter presents recent advances in molecular genetics, including marker-assisted selection, genetic relationship characterization, full genome description and genetic modification system. The chapter discusses the different expansion trajectories of several different regions in order to understand their causes. Finally, the chapter discusses opportunities for the future and provides a list of sources of additional information.