Abstract: Planck’s constant h, the tag of quantum theory, is one of the universal physical constants. The various distinct comings of Planck’s constant include the derivation of Plank’s spectral distribution formula for black-body radiation, Einstein’s interpretation of the experimental results concerning photoelectric effect, quantization of phase space by Planck (and also by Bohr and Bose), and Schrdinger’s effort to rescue the unified theory of gravity and electricity by Weyl, in which the role of the constant h is either simply a prefactor to make hν of the same dimension as energy or to make hν/kT dimensionless, a quantum of action per se,the volume element of the phase space or the angular momentum, or the energy quantum for light in the form of h , or a factor in the wave theory in the form of ih or i.The distinct comings of the Planck’s constant in the quantum theory rightly justifies its universality.By the way, neither Planck nor Einstein had introduced any revolutionary ideas by applying the energy quantum  h.

Key words: Planck’s constant, energy quantum of light, action quantum, quantization of phase space, phase factor