Sealing the tax system is the priority, and at the same time a major challenge for financial policy in many countries. In the European Union and Poland, numerous actions are taken to reduce the tax gap. The activities of businesses in the so-called shadow economy, their involvement in the fictitious turnover under the carousels of VAT abuse, and tax optimization associated with tax avoidance or evasion are considered the main reasons of the tax gap. Measurement of the scale of irregularities provides information on the effective functioning of tax regulations. The purpose of the paper is to determine the tax gap in Poland and European Union Member States (EU-28). The basic research method is to identify the activities of the tax administration aimed at limiting the loss of tax revenues in Poland, as well as to review the literature in this area. Based on the research, one can notice an improvement in that tax collection efficiency in Poland. This implies the need to continue reforming the activities of tax administration, which are crucial for the development, security of companies and credibility of economic processes.