Abstract: This article considers the extent of underemployment in the UK labour market. It begins by considering published unemployment and part-time work estimates, showing potential labour supply in the UK economy. Analysis is then presented which develops a measure of time-related underemployment using the Labour Force Survey (LFS) which looks at the additional hours that people in employment want to work. Further analysis describes recent trends in underemployment levels and rates, noting the changes that occurred as the UK economy contracted. The article also presents estimates of the volume of underemployment, in terms of the number of extra hours that underemployed people want to work, a useful indicator of potential capacity in the economy.