The number of superconductors known in 1935 was about 80, in 1950 over 100, and when last counted more than 900. Though the growth of a field cannot be measured by a single index, these numbers symbolize what has happened to the science of superconductivity; a sequence of discoveries has steadily enlarged the scope and importance of the subject. In 1950 the isotope effect (the dependence of superconducting transition temperatures on isotopic mass) was discovered, and subsequent advances have traversed the range from fundamental quantum theory to empirical rules for finding superconductors. In recent years there have been such discoveries as the superconductive tunneling effect, quantized magnetic flux, the electromagnetic structure of new types of superconductor, and tunneling supercurrents.