A universal basic income (UBI) would provide all citizens with a guaranteed income, irrespective of their earnings, age or household status. It would be financed from a flat-rate personal income tax. It would replace the existing work-based social security system with its plethora of benefit types, abatement rates and eligibility rules. However, when the trade-offs between the competing objectives of a tax/benefit scheme are considered, and the variety of individual and family circumstances that need to be addressed, the apparent simplicity of a UBI quickly disappears. The article shows that while the current tax/benefit system represents a ‘welfare mess’, and needs substantial restructuring, a UBI does not necessarily provide an adequate income for poverty relief, nor ensure labour force incentives, at an acceptable fiscal cost.