GST registration thresholds determine registration obligation and input tax credit eligibility, including compounding, interstate and reverse-charge cases. Registration under the proposed GST framework is driven by specified turnover thresholds and by the nature of supplies. Crossing the threshold or commencing a new taxable activity requires applying for registration within 30 days, with the effective date being the date of application; timely application preserves ITC from the application date while delayed application may preclude ITC for pre-registration purchases. A higher compounding turnover threshold permits eligible persons to opt into a scheme paying tax on turnover without entering the credit chain; inter-state suppliers and reverse-charge taxpayers must register irrespective of turnover.