The understanding of superconductivity has progressed in four stages. During the first period following the discovery of disappearance of electrical resistivity of mercury by Kamerlingh Onnes, attempts were made to understand superconductivity based only on zero resistance. The second period was initiated by the discovery of the Meissner effect. Based on new knowledge of the magnetic property of zero resistivity, phenomological theories were developed and new aspects of superconductivity were uncovered, culminating in the establishment of the BCS theory. The rediscovery of a second type of superconductivity together with flux quantization initiated the third period where fluxons play an active role. The fourth period, which is still in progress, was initiated by the discovery of oxides with dramatically high critical temperatures. All these activities were initiated by Kamerlingh Onnes' brilliant insight leading to the discovery of superconductivity.
