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A hundred years ago Max Planck introduced the quantum of action concept; the anniversary is celebrated in many ways all over the world. In the present paper we want to stress that the Planck constant h ‘initially introduced to account for some facts in a field of research which seemed a marginal one’ gradually showed its central role in the whole core of Physics. This paper, till now unpublished, was conceived in a former occasion and should be updated to take into account the main developments in macroscopic quantum phenomena and the role played by h in cosmology. However it could be useful to stimulate further contributions on these subjects.