To study the role of tax professionals, the paper presented in this ZEW Research Seminar merge tax records of the entire population of sole proprietorship taxpayers in Italy for seven fiscal years with the respective audit files from the tax revenue agency. Exploiting quasi-random variation in audit policy, the authors first document that there is a robust correlation between a taxpayer's evasion and that of the other clients of the same tax practitioner. The paper then exploits the structure of our dataset to study the mechanisms behind this phenomenon. The authors establish two results. First, taxpayers sort themselves into tax professionals with heterogeneous levels of tax evasion. Second, tax professionals generate informational externalities that affect their clients' tax compliance. The latter increases directly in response to personally experienced audits and indirectly following the audits of other clients of the accountant. While the direct effect of tax audits is decreasing over time, the indirect effect is increasing over time with a total cumulative marginal effect that amounts to 17% of that of the direct effect. The dynamic spillover generated by tax professionals is therefore an important channel of influence that ought to be considered in the evaluation and design of auditing schemes.