The phenomenon of superconductivity is a remarkable example of quantum effects operating on a truly macroscopic scale. The amazing properties of superconductors are related to the peculiar excitation spectrum of the superfluid. If one attempts to account for the rate of electromagnetic and acoustic absorption as well as the rate of nuclear spin relaxation in superconductors on the basis of a simple two-fluid energy-gap model, one quickly discovers inconsistencies. There are several direct ways of observing the energy gap in the elementary excitation spectrum of superconductors. In 1950 Ginsburg and Landau proposed an extension of the London theory which takes into account the possibility of the superfluid density varying in space. In a superconducting material, a finite fraction of the electrons are in a real sense condensed into a "macromolecule" which extends over the entire volume of the system and is capable of motion as a whole.