Input tax credit entitlement depends on invoice, receipt, tax payment and return filing; nonpayment within 180 days triggers reversal. Section 16 sets the conditions for claiming input tax credit: possession of a supplier-issued tax invoice or prescribed document, receipt of goods or services (including deemed receipt in prescribed circumstances), supplier furnishing of outward-supply details and no restriction on communicated ITC details, actual payment of tax (in cash or through admissible ITC), and filing the return under section 39; credit on goods received in instalments accrues on the last lot, nonpayment to supplier within 180 days triggers reversal with interest, depreciation on the tax component of capital assets disqualifies ITC, and claims are time barred by the thirtieth day of November following the financial year subject to specified exceptions.