Show cause notice can trigger assessment, interest and a specified penalty unless tax is paid promptly. Determination of unpaid, short-paid or erroneously refunded tax and wrongly availed or utilised input tax credit is initiated by a show cause notice requiring the person chargeable to explain why tax, interest under section 50 and a penalty should not be recovered. The proper officer must issue the notice at least three months before the assessment time limit and may serve a statement extending the same grounds to other periods. Voluntary payment of tax and interest before notice prevents issuance of that notice in respect of the amount so paid, while payment within thirty days of the show cause notice bars penalty; the officer must issue a final order determining tax, interest and a specified penalty within three years from the relevant return due date or erroneous refund.