The present review describes the detail of recent development of phosphonate reagents and experimental methodologies to achieve high Z-selectivity in the Homer-Wadsworth-Emmons (HWE) reaction. While the general HWE reagents with common alkylphosphono groups produce thermodynamically favored E-olefins, several modified phosphonate reagents successfully give Z-olefins in the high selectivities. The central device for this goal is the introduction of the electron negative alkoxy or aryloxy groups to the phosphorus moieties. These synthetically useful phosphonate reagents, their reactions, the origin of the selectivities, and theoretical studies are presented.