Photobiomodulation (PBM) is a modulation of laser irradiation or monochromatic light (LI) on biosystems, which stimulates or inhibits biological functions but does not result in irreducible damage. LI might be of low intensity LI (LIL) (about 10 mW/cm 2 ), or moderate intensity LI (MIL) (10 2 -10 3 mW/cm 2 ). PBM of LIL or MIL (LPBM or MPBM) is studied from the homeostatic viewpoint in this paper. Homeostasis is redefined as the function-specific homeostasis (FSH), a negative-feedback response of a biosystem which maintains the functionspecific conditions inside it. PBM is classified into two kinds, the FSH-specific PBM (fPBM) and developmental PBM (dPBM). For fPBM, there is no PBM of LI on the function in FSH, but there is PBM of LI on the function far from FSH. dPBM can disrupt FSH. It can be found that LPBM is an fPBM, and whether MPBM is fPBM or dPBM depends on MIL dose and cell sensitivity. Low level LI therapy is just clinical applications of fPBM, so that it is a cellular rehabilitation.