Article 18 imposes the return filing obligation under Chapter 7 of the Decree-Law. It requires the Taxable Person to submit a Tax Return to the Authority at the end of each Tax Period. The Article does not itself prescribe the deadline or the manner of filing; the return must be submitted within the timeframes and in accordance with the procedures specified in the Executive Regulation of the Decree-Law. The obligation attaches to every Taxable Person for each Tax Period, so that the Authority receives the information and data specified for Tax purposes on the form it prepares, period by period.