Cambodia has international airports at Phnom Penh and Siem Reap.
Direct flights connect Phnom Penh International Airport with China, Laos, Malaysia, Singapore, South Korea, Taiwan, Thailand and Vietnam.
Direct flights connect Siem Reap
Angkor International Airport with Laos, Malaysia, Singapore, South Korea, Taiwan, Thailand and Vietnam.
Travelers going specifically to visit the Angkor temple ruins may prefer to use Siem Reap as it's only a few minutes away from the main sites; however as Bangkok Airways has a monopoly on direct flights between Bangkok and Siem Reap, it's a lot cheaper to fly to Phnom Penh and to take the bus (or cross overland from Bangkok).
Low-cost carrier Air Asia has introduced flights from Kuala Lumpur and Bangkok to Phnom Penh and Kuala Lumpur to Siem Reap, while Jetstar Asia has begun flying from Singapore to Siem Reap and Phnom Penh.
Other airlines operating flights to/from Cambodia
Asiana Airlines, Bangkok Airways, China Southern Airlines, Dragonair, Eva Airways, Korean Air, Lao Airlines, Malaysia Airlines (MAS), Shanghai Airlines, Siem Reap Airways (a subsidiary of Bangkok Airways), SilkAir, Singapore Airlines, Thai Airways International and Vietnam Airlines.
All six international border crossings are open 7 AM to 8 PM and all have visa-on-arrival facilities at the Cambodian immigration checkpoints.
On the Thai side, there are good sealed roads to all 6 crossings. There are no direct bus services from Thailand that cross the border, but there are buses to all crossings except Chong Sa-Ngam.
On the Cambodian side, more basic and less frequent bus services are available from Poipet and Koh Kong.
To Siem Reap and Battabang
The busiest land crossing into Cambodia, and the most convenient for onward travel to Siem Reap and the Angkor Archaeological Park. Along the stuff of nightmares, the road has finally been paved all the way from the border to Sieam Reap and Phnom Penh.
To Koh Kong & Sihanoukville
Take a bus to Trat (transfer in Chanthaburi if necessary) and from there a minibus to the border. After crossing into Cambodia there are two possibilities - the once-daily boat to Sihanoukville (can be unpleasant in adverse weather), or else a minibus or taxi to either Sihanoukville or Phnom Penh.
Through bus tickets cost ~USD$10-12. Bus passengers must walk between the Cambodian and Vietnamese checkpoints and transfer between the two buses that travel the Cambodian and Vietnamese legs respectively. At this crossing only one passport photo is required for a Cambodian visa.
Alternatively, it can be interesting to join a Mekong Delta tour (2-3 days) that travels between the two cities; the total cost is about US$25-35 for three days.
Mai Linh Taxi Company in Ho Chi Minh City provides bus from Ho Chi Minh City to Siem Reap directly from beginning of 2007. The price is US$18.
It is possible to cross at the Trapeang Phlong - Xa Mat border but as of now you have to organise your own transport.
Onward transportation not regularly available. Cambodian visas available, but expect 1$ to 2$ of transaction fees on both sides of the border. Travel agencies on both sides have border crossing packages.
There is one border crossing for tourists on the Mekong, a 90 minute speedboat ride north of Stung Treng. The border guards have few opportunities for "alternative" income, and will usually try to make a few extra dollars from scamming tourists.
There are no ferry services between Cambodia and Thailand. However it is possible to cross the border from Thailand to Koh Kong on foot, take a 15 minute share taxi or motodop (motorbike taxi) ride, and then proceed to Sihanoukville by ferry from there (or vice-versa).
It's possible to travel between Ho Chi Minh City and Phnom Penh by boat, or by combination of road and boat. Fast boats leave daily from Chau Doc in Vietnam's Mekong Delta and take 5 hours to reach Phnom Penh. Chau Doc is a four hour drive from Ho Chi Minh City. A popular overland route is to make a three day trip, stopping at Can Tho and Chau Doc before taking the boat to Phnom Penh.