 Low-level light therapy, LLT, has been used for over 40 years to reduce pain, inflammation, and edema, promote healing of wounds, deeper tissues, and nerves, and prevent cell death and tissue damage. Despite positive findings from experiments in vitro, animal models, and randomized controlled clinical trials, LLT remains controversial in mainstream medicine due to the complexity of rationally choosing amongst a large number of illumination parameters and the publication of both positive and negative studies. A biphasic DOS response has been observed where low levels of light have a better effect on stimulating and repairing tissues than higher levels of light, which is described by the Arnt-Schultz curve. This review will cover the molecular and cellular mechanisms in LLT and describe recent results that provide scientific explanations for the biphasic DOS response. This article was authored by Yingying Huang, Aaron Chien, James D. Carroll, and others.