The close relationship between superconductivity and ferromagnetism is discussed and illustrated; it is suggested that several mechanisms cause superconductivity and also ferromagnetism. Various rare earth alloys show simultaneous superconductivity and dilute ferromagnetism. Such dilute ferromagnetism does not occur, however, in the transition-metal superconductors. Striking differences in the effects of adding magnetic elements to transition-metal superconductors are traced to the presence or absence of localized magnetic moments; such occurrences show a dependence on electron concentration similar to superconductivity. Superconductivity in the transition and nontransition elements shows a simple dependence on valence electron concentration—but when such elements are mixed they interfere drastically.