Good tax agents, third parties paid by taxpayers to act on their behalf in their dealings with HM Revenue & Customs, help their clients get their tax right. But, self-assessed income tax returns filed by customers represented by agents are more likely to have under-declarations of tax (resulting from error, failure to take reasonable care or evasion) than returns filed by non-represented taxpayers. A key reason may be that the tax affairs agents deal with are more complex. However, analysis indicates that paying for professional help is not without risk for a taxpayer and that there might be an opportunity for HMRC to increase tax revenues by providing better support to tax agents and by better targeting of poorer ones. A three per cent reduction in the average amount of tax under-declared by represented taxpayers could lead to over £100 million extra revenue each year. At present, lack of data on individual tax agents prevents the Department's taking a tailored approach to its dealings with agents and providing feedback on performance. With better use of data, HMRC could make more targeted interventions based on risk and achieve greater value for money. HMRC has recognised the importance of developing its relationship with tax agents and has taken steps to work more effectively with this group. Initiatives have included the introduction of a priority telephone line for agents' queries on self-assessed income tax and PAYE. The Department has also encouraged tax agents to file tax returns online.