The tax and benefit system can have dramatic effects on financial incentives to work at all and, if so, how much. IFS researchers do a great deal of work quantifying those incentives and the effects that actual and potential reforms have on them. We have also examined how people respond to those incentives in practice. This includes directly estimating the impact that tax and benefit reforms have had on people’s labour supply and taxable incomes. It also includes using models of labour supply behaviour estimated from previous reforms to assess the possible impact that future reforms might have.