The first part of this chapter provides an overview of what lay behind record productivity growth in the US economy between 1929 and 1941. The second part considers the role of rigidities and other negative supply conditions in worsening the downturn and slowing recovery. While arguing consistently that the overarching explanation of the Great Depression will and should continue to emphasise a collapse and slow revival in the growth of aggregate demand, the chapter spends relatively little time on what drives this. The emphasis of the chapter is on aggregate supply—both the broad array of positive shocks that propelled potential and, eventually, actual output forward, and the negative conditions which, in interaction with aggregate demand, may have increased the size of the output gap and prolonged its persistence. An appendix offers discussion and updated calculations of productivity growth rates for the critical period 1929–41.