Getting to downtown

The easiest: get your hotel to pick you up.

Many hotels offer free taxi services from the airport when you arrive. Check with them before your arrival and they will meet you with your name written on a piece of paper. Just get into the car and enjoy the ride (takes about 45-60 minutes into town, depending on how heavy the traffic is and where your hotel is). 

Even if they don't offer you a free pickup, they are often very competitively priced.  Allowing them to arrange it for US$15-20 is money well spent.

The commonest: Taxi

When exiting Noi Bai International Airport, visitors will see the taxis lined up against the curb.  There isn't really a taxi line as such just pick a cab you like or wait for more to come.  There used to be numerous signs giving the fixed price: VND 350,000 ($16) for a sedan and VND 380,000 for a MPV (7 seater, Toyota Innova).  However, these have gone, so you can either negotiate a price or pay by the meter.

There are several companies operating this route below:   Visitors will pay directly to the driver when arriving at hotel. No toll fee or any other charge. Note that the per km meter rate varies from company to company in a range of 12,000 to 12,800/km (Except for TaxiGroup which is 15,400/km -- i.e., 20-25% more). The final price can be anything between VND325,000 and VND475,000 depending on where you go and which brand of taxi you take.

 Here are some links (but this is not a complete list).

Note: it is a good habit to write down the number plate of the taxi in case the driver want to cheat you.

By Minibus

Continue to the right past the taxis and you will find the minibuses:

Airport Minibus one way trip costs VND40,000 ($2). Minibuses leave the airport when they are full.

You can also hire a whole minibus for VND500,000. 

They will manage to squeeze you and your luggage in somehow, but prepare to be squashed.

Public Bus

Keep going past the minibuses and through the car park and you will find the public buses:

Bus #7 (Cost VND8,000) connects the airport and Cau Giay bus station (in the West of the city).

Bus #17 (Cost VND9,000) will get you to the Long Bien bus station (near the Old Quarter).

It can be crowded and you will need to transfer from bus station to your hotel.  They may not let you on if you have more than a small bag, or they may charge you for two seats.