Assessment of unpaid tax: notice-based recovery with interest, penalties and time limits, subject to hearing and procedural safeguards. Determination of unpaid, short paid, erroneously refunded tax or wrongly availed input tax credit follows separate procedures for non-fraud and fraud cases: the proper officer issues a show cause notice or deemed statement detailing tax, interest and penalty; taxpayers may preempt notices by payment of tax with interest (and specified penalty in fraud cases); timely payment after notice or order concludes proceedings; the officer determines amounts after representations and must issue an order within applicable statutory limitation periods, with procedural safeguards including hearing rights, limited adjournments, exclusion of judicial stay periods, and modification of interest and penalty proportionate to any appellate change.