Since the discovery of light amplified by stimulated emission of radiation ??? Lasers ??? in 1967, the effects of light on the improvement of tissue healing, promoting pain relief, decreasing inflammation and swelling, as well as preventing further tissue damage, have been known and constantly studied. Due to these useful properties for health, the interest of researchers for photobiomodulation has been growing, especially with the advent of light-emitting diodes and other light sources with various wavelengths. There are many known effects of light at the molecular level, and most of them occur in the mitochondria (especially by the interaction of light with cytochrome-C oxidase), such as increased ATP production, differential gene expression due to the induction of transcription factors, and altered oxidative status due to the modulation of oxidative stress. This chapter provides up-to-date information regarding those molecular and cellular mechanisms of low-level light according to wavelength, light dose, and other parameters, as well as some of its consequences in the cellular and tissue levels. The concept of the Arndt-Schulz curve, regarding the biphasic dose response to low-level light, is also explained.