Alvin Hansen was instrumental in the development of American Keynesianism, social security, and full-employment policies. His was a search for practical policies that could reconcile the demands of technological progress and private enterprise with employment security and a better society. How could government policy best support a dynamic maturing economy in the face of challenges from the lack of population and territorial growth that had supported the American expansion of the nineteenth century? What was a suitable stabilisation for the new frontiers of the twentieth century shaken by war and the Great Depression? These themes, together with a wider view of Hansen’s contributions to the development of macroeconomics between and after the two world wars, are examined in this, the first, major intellectual biography to focus solely on Hansen.