Over the past decade, light-emitting diodes (LEDs) have been adopted for use in various liquid crystal display (LCD) devices, from mobile phones to LCD televisions. LCDs with LED backlight units have become a popular choice for display purposes. For LCD devices, the quality and features of the light source affect or even directly determine the optical quality and market competitiveness of the LCD products. Several advantages of LEDs solve a few outstanding problems with LCDs. LEDs inside LCDs have solidified the mainstream status of LCDs. We describe the types and structures of LED LCD backlight units, the market trends and technological developments, the advantages and key technologies of LED LCD televisions, and the key points in the optical design of LED backlights.