A substantial number of employees work additional non‐contract hours for no pay. We advance several economic explanations for this phenomenon. Empirical work is based on the UK Labour Force Survey for 1993/94. We establish the quantitative importance of unpaid work on overtime hours and hourly earnings for both males and females. We demonstrate, via Mincer wage growth equations, that accounting for unpaid work leads to significantly revised estimates of returns to education, experience and tenure. We test a number of hypotheses arising from our economic discussion using Tobit regressions of unpaid overtime. For comparative purposes, we also estimate equations of paid overtime.