- The present status of the theory of Coulomb displacement energies, Delta Ec, of analogue states in nuclei (mirror nuclei in particular) is reviewed. In recent years, a considerable amount of investigation has been carried out on the problem of evaluating Delta Ec in order to resolve the persistent discrepancy between theory and experiment over a wide range of nuclei. The calculated values are found to be approximately 7% smaller than the experimental values. This discrepancy is known as the Okamoto-Nolen-Schiffer anomaly. The authors review in this work all possible correction terms which contribute more than 10 keV to Coulomb displacement energies, and also discuss and suggest the (possible) solution of the Coulomb energy problem in nuclei.