Changes in tax auditing procedures, tax administration and generally in tax policy have attracted the attention of researchers in recent years. The aim of this study is to examine the relationship between tax audit effectiveness, tax legislation and the use of specialized information system tools. We use the ability of public tax auditors to track tax infringements as a measure of tax audit effectiveness. Over two hundred structured questionnaires were constructed and distributed to tax auditors, who work in Greek public taxation agencies. Factor Analysis and multiple regression analysis were employed in order to examine our hypotheses. The results demonstrate that the use of information system tools can enable tax auditors to track properly tax infringements, thereby contributing to increased tax audit effectiveness. It is also suggested that constant changes in tax legislation inhibit tax auditors from being effective in their work. Our results call for direct policy intervention, including simplification of tax legislation and better training of tax auditors in the use of information systems. Key words: Tax audit, auditing methods, tax infringements, information systems, tax legislation.