At Delhi's Indira Gandhi International Airport (DEL), the requirement for immigration depends on your flight path. If you are on an International-to-International connection and both flights are on a single ticket (one PNR) with luggage checked through, you generally stay "airside" and do not clear immigration; you simply follow the "International Transfer" signs to a security check. However, if you are transitioning from an International to a Domestic flight (or vice versa), you must clear immigration and customs at Delhi, as it is your first point of entry into India. Even for international-to-international transfers, if you have booked two separate tickets or need to collect and re-check bags, you will likely need to pass through immigration (requiring a valid visa or transit visa) to access the check-in counters. Always confirm your terminal, as international flights operate from T3, while domestic flights may use T1, T2, or T3, requiring a landside transfer.