Photobiomodulation (PBM) is a biomodulation of laser irradiation (LI) or monochromatic light. The LI may be low intensity LI (LIL) at about 10 mW/cm2, moderate intensity LI (MIL) at 0.1~1.0 W/cm2 or high intensity LI. The progress in PBM studies is reviewed from homeostatic viewpoint in this paper. Function-specific homeostasis (FSH) is a negative-feedback response of a biosystem to maintain the function-specific conditions inside the biosystem so that the function is perfectly performed. FSH can resist internal or/and external disturbance under threshold, but can be disrupted by FSH-specific stress (FSS). There is a FSS-specific homeostasis (FSSH) in which a FSS not only disrupts the original FSH, but also establishes a new FSH so that it is called a successful stress. Low level LI (LLL) has been so defined that it cannot modulate a successful stress or a function in its FSH, but it can modulate a FSS far from its FSSH. LIL might be coordinately mediated by many kinds of proteins in membrane of cells or cellular organellae so that the FSH can be self-adaptively established. MIL might be mediated by endogenetic photosensitive proteins. LLL can be used to diagnose the process for a function to establish its FSH or for a FSS to establish its FSSH and the FSH or FSSH itself. The applications of PBM in enhanced recovery after surgery are also discussed.