Central banks have resorted to various unconventional monetary policy tools since the onset of the Global Crisis. This column focuses on the macroeconomic effects of the Federal Reserve’s large-scale purchases of mortgage-backed securities – in particular, through reducing the ‘mortgage spread’ between interest rates on mortgages and government bonds at a given maturity. Although large-scale asset purchases are found to have substantial macroeconomic effects, they may not necessarily be the best policy tool at the zero lower bound.