Entering India at Delhi's airport is relatively straightforward, with standard immigration and customs procedures.
Non-passengers are not allowed to enter the Departures building so when leaving Delhi you need to show your aeroplane ticket (a digital copy is OK) in order to enter the terminal. If meeting arriving passengers you can pay ₹100 to enter the Arrivals building.
Technically you’re supposed to declare Indian rupees in excess of ₹10,000, any amount of cash over US$5000, or a total amount of currency over US$10,000 on arrival.
You're also prohibited from importing more than one laptop, 2L of alcohol, 100 cigarettes or equivalent, or gifts and souvenirs worth over ₹8000.
To protect India’s cultural heritage, the export of certain antiques is prohibited, especially those which are verifiably more than 100 years old. Reputable antique dealers know the laws and can make arrangements for an export-clearance certificate for old items that are OK to export. Detailed information on prohibited items can be found on the Archaeological Survey of India website (http://asi.nic.in).
To enter India you need a valid passport and an onward/return ticket, and a visa. Note that your passport needs to be valid for at least six months beyond your intended stay in India, with at least two blank pages. If your passport is lost or stolen, immediately contact your country’s representative. Keep photocopies or digital copies of your airline ticket and the identity and visa pages of your passport in case of emergency.
Most nationals can stay for up to 60 days with a hassle-free, double-entry, e-Visa (www.indianvisaonline.gov.in/evisa). Longer stays (up to six months) require a standard tourist visa.
Apart from citizens of Nepal, Bhutan and Maldives, who don't need visas for India unless they are arriving from mainland China, and citizens of Japan and South Korea, who can obtain a visa on arrival, everyone needs to apply for a visa before arriving in India.
However, over 100 nationalities can obtain the wonderfully hassle-free, 60-day, double-entry, e-Visa (www.indianvisaonline.gov.in/evisa), which you apply for online at least four days prior to your planned arrival; you'll quickly receive a digital copy of your authorisation, which you need to print out and bring with you to your airport of departure; this visa is valid for 60 days from the day you arrive in India, but note that you can only get an e-Visa if you are arriving at one of the 26 listed airports or five seaports. Delhi is, of course, one of the listed airports.
For longer trips, you'll need to obtain a standard six-month tourist visa, which is valid from the date of issue, not the date of arrival in India. Once in Delhi, If you need to register your visa (for stays of more than 180 days), or need a visa extension (there has to be a very good reason) or a replacement for a lost passport (required before you can leave the country), then you should apply online first (https://indianfrro.gov.in/eservices/home.jsp). If you need to see someone in person about your visa issue, then it will be at the Foreigners’ Regional Registration Office.