This paper provides a new analysis of the main stylised facts underlying the evolution of labour supply at the extensive and intensive margins in three countries: the United States, the United Kingdom and France. We propose a definition of the extensive and intensive margins corresponding respectively to the employment rate and to hours when employed. This definition is robust to the choice of the reference period and we develop a new statistical decomposition that provides bounds on changes at these margins. We focus on longer-run labour supply changes over the period 1977 to 2007 and abstract from the shorter-run impact of recessions. Examining secular changes over this period, we show that both margins matter in explaining changes in total hours. We then provide a detailed analysis across countries and across time by demographic type. Given the large systematic differences we uncover in the importance of these margins by age and gender, it is unlikely that a single explanation will suffice to account for the macroeconomic evolutions in the three countries.