The labour force participation rate in the US has fallen dramatically since 2007. This column traces this decline to three main factors: the ageing of the population, cyclical effects from the Great Recession, and an unexplained portion, which might be due to pre-existing trends unrelated to the first two. Of these three, the ageing of the population plays the largest role since it is responsible for half of the decline. Taken together, these factors suggest a roughly stable participation rate in the short-term, followed by a longer-term decline as the baby boomers continue to age. However, policy can play a meaningful role in mitigating this trend.