Federal political systems proliferated during the twentieth century, until by the end of the century some 24 countries called themselves federations or met the usually accepted criteria for a federation. Over the course of the century, the popularity of the federal solution varied between periods of skepticism and extensive development, however. The period between 1945 and 1960, and again in the last decade, were particularly marked by extensive federal developments. An assessment of developments over the course of the century has led to the identification of some positive aspects regarding the effectiveness of federal systems. This experience has also taught us a number of major cautionary lessons about the basis required for effective federalism. A notable result of the analysis was the great number of variations and even innovations in the application of the federal idea.