Photobiomodulation is an effect of low intensity monochromatic light or laser irradiation on biological systems, which stimulates or inhibits biological functions but does not results in irreducible damage. However, many cellular experiments, animal experiments or clinic studies with monochromatic light or laser irradiation which intensity is not so high that it directly destroys cells have found no photobiomodulation but cell compartment damage, apoptosis or necrosis so that a new concept, a moderate intensity laser or monochromatic light, should be defined if its intensity is so high that no photobiomodulation can be observed, but is so low that it can not directly destroy cells. There might be two pathways mediated the effects of a moderate intensity laser or monochromatic light: one is mediated by the photodynamic effects of endogenetic photosensitizers to induce apoptosis or necrosis, one is mediated by laser or monochromatic light induced stress wave through endogenetic chromophore to induce apoptosis. These effects can be used to understand laser or monochromatic light induced cell compartment damage, apoptosis or necrosis. They are beneficial in phototherapy of cancer and scar, but deleterious in phototherapy of delayed onset of muscular soreness.