Input Tax Credit eligibility requires registration, prescribed invoices, receipt of supplies, tax payment or valid ITC utilisation, and timely returns. Section 16 entitles a registered person to claim input tax credit for inputs used in the course or furtherance of business, subject to conditions: possession of prescribed tax documents issued by a registered supplier, supplier's furnishing of outward-supply details and communication to the recipient, receipt of goods or services (including deemed receipt rules), payment of tax to the Government or valid utilisation of ITC, and filing of prescribed returns. Time limits and transitional relaxations, plus further restrictions under section 17(5) and Rule 36, affect admissibility.