The interaction of photons with cells is a necessary and essential condition for photobiomodulation to occur. Absorption and transduction of this energy must occur and it is well known that cellular molecules and structures are capable of absorbing this energy at various wavelengths. It is also known that transmission of some portion of the incident light occurs depending upon the wavelength, the irradiance, the time course of the interaction, and the particular tissue being exposed to the beam. This interaction is dubbed a ‘‘cold’’ effect because the primary photobiomodulatory event is nonthermal, at least to the extent that the temperature of the target tissue is not elevated or is minimally elevated over baseline. Such measurements are inevitably affected by tissue perfusion and are not easily measured at the level of an individual cell. However, temperature clamping experiments have documented that the biostimulatory effects of 830-nm photoradiation occur if the tissue temperature is held constant. 1 Although it is likely that a number of intracellular molecules are capable of transduction of light energy into a useful form for eukaryotic cells, the cytochrome system is a primary target. Several investigators have documented that photobiomodulation upregulates ATP production via this system. 2–4 This is the good news. However, the naysayers would point out the fact that