We discuss light-induced stimulation and inhibition of biological activity by means of three types of competing processes. In the visible region, these competing processes are the formation by photosensitization of reactive oxygen species (ROS) which stimulate the redox activity of the respiratory chain (RC) on the one hand, and intramolecular electronic-vibrational energy transfer from an endogenous photosensitizer to an enzyme of the RC, thereby bringing this enzyme into an inactive configuration and paralyzing the RC, on the other hand. Moreover, there is competition between stimulation of the redox activity of the RC by the ROS and a slower process where the enzymes of the RC react with the ROS, again paralyzing the RC. This paralysis of the RC plays a dominant role in photodynamic therapy, where exogenous photosensitizers together with a sufficiently large visible light-energy dose lead to overproduction of ROS. Finally, in the far-red region, there is competition between reactivation of the ATPase ion pumps in the cell membrane and inhibition of the enzymes in the RC as a result of vibrational overtone excitation. Photobioactivation is shown to lead to enhanced transient Ca 2+ concentration increase (calcium oscillations) in the cytosol, thereby triggering further biological activity such as afflux of intercellular messengers which open gated ion channels in neighboring cells, producing calcium waves. Addition of ROS scavengers or quenchers such as SOD in the presence of catalase neutralizes photobiomodulation induced by visible light.