We use administrative tax data from audits of self-assessment tax returns to understand what types individuals are most likely to be non-compliant. Non-compliance is common, with one-third of taxpayers underpaying by some amount, although half of aggregate under-reporting is done by just 2 per cent of taxpayers. Third-party reporting reduces non-compliance, while working in a cash-prevalent industry increases it. However, compliance also varies significantly with individual characteristics: non-compliance is higher for men and younger people. These results matter for measuring inequality, for understanding taxpayer behaviour, and for targeting audit resources.