One of the great unsolved problems of science and also physics is the prediction of the three dimensional structure of a protein from its amino acid sequence: the folding problem. It may be stated that the deep connection existing between physics and protein folding is not so much, or in any case not only, through physical methods (experimental: X–rays, NMR, etc, or theoretical: statistical mechanics, spin glasses, etc), but through physical concepts. In fact, protein folding can be viewed as an emergent property not contained neither in the atoms forming the protein nor in the forces acting among them, in a similar way as superconductivity emerges as an unexpected coherent phenomenon taking place on a sea of electrons at low temperature. Already much is known about the protein folding problem, thanks, among other things, to protein engineering experiments as well as from a variety of theoretical inputs: inverse folding problem, funnel–like energy landscapes (Peter Wolynes), helix–coil transitions, etc. Although quite different in appearance, the fact that the variety of models can account for much of the experimental findings is likely due to the fact that they contain much of the same (right) physics. A physics which is related to the important role played by selected highly conserved, “hot”, amino acids which participate to the stability of independent folding units which, upon docking, give rise to a (post–critical) folding nucleus lying beyond the highest maximum of the free energy associated to the process.