The protein folding-unfolding equilibrium is governed by a large number of non-covalent interactions under the constraints imposed by the covalent backbone. We have developed a method based on protein reconstitution from fragments that allows us to measure the relative contribution of different kinds of non-covalent interactions to protein assembly. Our results show clearly that interactions among hydrophobic side chains are by orders of magnitude more important than Coulombic interactions in protein folding and assembly, while repulsive electrostatic interactions play an important role in folding specificity by disfavoring unwanted conformations.