There are now direct flights from Paris, Los Angeles, San Francisco, as well as most cities in East and Southeast Asia to Hanoi. The airport- Noi Bai International Airport or HAN- is located some 45 kms outside of the downtown areas, which is translated into about an hour taxi ride.
Vietnam Airlines remain the main carriers of domestic flights and many international flights. Some budget airlines also have frequent flights in and out of Hanoi, such as AirAsia (to Thailand and Malaysia), Tiger Air (to Singapore).
From North America, most indirect flights have a transit in either Hongkong, Tokyo, Osaka or Seoul.
From Europe, there might be transits in Paris, Hongkong, or Moscow, depending on your starting points.
Hanoi is served by two main bus terminals- in the North and in the South, that receive and send bus to almost any provinces in Vietnam. Open tour buses usually drive you to the downtown area without stopping at the terminals. Check out Giap Bat and My Dinh bus station.
Hanoi train station is about 10 minutes by taxi from the city center. If you cross the border to Vietnam from China, it is likely that you will board the train in Laocai and arrive in Hanoi in early morning.
Between the Airport and City Center:
You can take the Vietnam Airlines minivan into town, which drop you off at Vietnam Airlines office in Hang Bai street in downtown. This service costs approximately $2 and has frequent service during the day. From downtown to the airport, you can use the same services, which depart almost hourly from the office until 6:30 p.m. Please take traffic into account and allow plenty of time to arrive at the airport.
The cheapest, but may not be the most convenient option is to take a local bus. When you arrive, walk outside of the airport to the far right. You will see a group of bus in yellow and red, number 7. This bus reaches the airport every half hour. Hop on one and get off the bus at Daiwoo hotel and take Bus number 9 to its terminal in the Old Quarter. If you have a lot of luggage and not ready to experience local life, taking this bus is not the best choice.
Taxis to the airport from town come at a slight discount if you bargain, usually costing about $10. From the airport, it can cost up to 12$. Only official taxis are allowed to park outside the airports and someone with a walkie-talkie will quickly approach you to offer one. Prices are quite homogenous. If you want a better bargain, walk outside of the airport, wave for a street taxi and bargain. In general, taxi in Vietnam is quite safe.
Outside of the airport, especially closed to the bus 7 parking lot, are a crowd of motor-taxi driver who readily offer you and your luggage a ride to the center. It costs about 7 dollars for such trip. However, this is not the safest choice so do not risk your life here. You can always try riding the motor when you are inside the cities.