Proper lighting is one of the most important aspects of any reef aquarium. The zooxanthellae that lives in coral tissues and gives them much of their color, depends heavily on natural sunlight to flourish in a symbiotic relationship with the corals. The lighting we provide for the aquarium needs to simulate this as closely as possible. There are many options when choosing a light source for the reef aquarium which can be confusing. What are the benefits of one system such as HO, VHO, T5, T5HO, and HQI over another? How much lighting is required for for proper coral health? What is do all these terms mean, 10,000K, 420nm, lux, and lumens? These are terms used when touting lighting fixture specifications, which we will explain in more detail. Lux is a derived unit based on lumen, and lumen is a derived unit based on candela(foot candle). One lux is equal to one lumen(a measure of the intensity of light) per square meter. To simulate the lighting for the reef aquarium we need to understand the light that occurs naturally on the reef with sunlight. The ocean has a blue color because it is the blue part of the spectrum that penetrates the water the deepest and the red and yellow are filtered out. Corals found in shallow reefs and tide pools receive more of the full spectrum and intensity of natural sunlight, while corals found in deeper parts of the reef receive more of the blue spectrum and less intense light. The wavelength of light is measured in nanometers, with violet and blue at the lower end of the visible spectrum 320nm and the red is at the higher end at 700nm. Coral health requires photosynthesis in the zooxanthellae which intakes carbon dioxide and produces oxygen and carbohydrates for the corals.