The chapter begins by identifying the theoretical roots of labour market concepts, notably the Phillips Curve relating unemployment and inflation. It then presents the definitions of “employment” and “unemployment” developed by the International Labour Organisation. These are measured by the quarterly Labour Force Survey, which provides not just simple counts but also flows between these categories, here presented graphically. One problem is that localised unemployment data use different definitions from the national headline rate, but a larger problem is that in all measures individuals must be counted as either employed or unemployed, when increasing numbers of workers work fewer or more hours than they wish, sometimes on variable hours contracts or as insecure sub-contractors in the “gig economy”. These new forms of work, generally disadvantaged, make gathering reliable data harder, and the chapter ends by discussing earnings data, and measuring the impact of minimum wage legislation.