Determination of unpaid tax requires a show cause notice; timely payment with interest can avoid penalty. Determination of unpaid, short paid, erroneously refunded tax or wrongly availed input tax credit (other than by fraud or wilful misstatement) is effected by serving a show cause notice requiring payment of tax, interest under section 50 and a penalty; the officer may also serve a statement for additional periods on the same grounds. Voluntary payment of tax and interest and written intimation prevents notice for the paid amount unless deficient; payment within thirty days of the show cause notice avoids penalty. After representations, the officer determines tax, interest and penalty and issues an order within the prescribed limitation period.