 Low-level light therapy, LLT, is a medical technique that uses visible or near-infrared light to reduce pain, inflammation, and edema, promote healing of wounds, deeper tissues, and nerves, and prevent cell death and tissue damage. Despite many positive findings from experiments conducted in vitro, animal models, and randomized controlled clinical trials, LLT remains controversial and mainstream medicine due to the complexity of rationally choosing amongst a large number of illumination parameters. A biphasic dose response has been observed where low levels of light have a better effect on stimulating and repairing tissues than higher levels of light. The molecular and cellular mechanisms underlying LLT are incompletely understood, but recent studies provide scientific explanations for the biphasic dose response. This article was authored by Yingying Huang, Aaron C. H. Chin, James D. Carroll, and others.