Cambodia is served by an increasing number of flights from neighboring countries to both Phnom Penh and Siem Reap, though the best choice is from Bangkok in Thailand. There are now five overland crossings open to foreigners, two from Thailand, and two from Vietnam and one from Laos. Even if you have obtained a Cambodia visa before entry, it is essential to obtain an entry stamp in your passport when crossing overland, as failure to do so will cause serious problems when you come to leave the country.
From Bangkok, there are regular daily flights to Phnom Penh, taking around an hour, with Thai Airways, Bangkok Airways; the last of these offers slightly cheaper fares than the other two, but can't be booked from outside the region. Bangkok Airways and Siem Reap Airways also fly daily to Siem Reap, with slightly higher frequency in the high season of December to February. The Cambodia Angkor Airways (National Ariline) will be operating this flight soon.
Overland trips to Cambodia from Thailand have increased in popularity and are well publicized in Bangkok, particularity on the Khao San Road, where travel agents try to sell their Bangkok-Siem Reap trips by alleging that doing the trip independently entails various problems (dealing with Cambodian border officials, sorting out onward transport, etc). In fact, it's straightforward enough to do the journey by public transport, and the convenience of using one of these private firms can be offset by much waiting around until the required number of passenger's turns up. Though most of these companies are reputable, a small minority of travelers has reported being ripped off over visas, and even being left for hours at the border waiting for onward transport; therefore it's worth asking fellow travelers of staff at your guesthouse about companies they would recommend or avoid.
The Aranyaprethet/Poipet border crossing is ideal if you want to start your visit to Cambodia in the north at Battambang and Siem Reap, while Trat/Koh Kong is good for Sihanoukville and Phnom Penh. From Bangkok, you can reach Aranyaprathet by train (7hr) or by air-con bus (4hr); there are also air-con buses to Trat 95hr). Both borders are open daily (7am-5pm) and visas are issued on arrival. From Poipet, onward transport by shared taxi or pick-up is readily available to Sisophon ( for Siem Reap) and daily boats from Hong Kong to Sre Ambel (for Phnom Penh) and to Sihanoukville. Poipet is in fact derelict, the nearest train station being at Sisophon.
Cambodia-Thailand crossing border:
There are several regular daily flights to Phnom Penh and to Siem Reap from Ho Chi Minh City, operated by Vietnam Airlines and Royal Phnom Penh Airways. Border crossing are open to foreigners at Moc Bai/Bavet, 200km southeast of Phnom Penh, and at Chau Doc on the Bassac River, through note that Cambodian visas are not issued at either crossing point. From Bavet, it's easy to get shared taxis to Phnom Penh (6hr); though the road has been in appalling condition, the journey time should be reduced when repairs are completed at the beginning of 2003. If you've crossed over at Chau Doc, you may be able to get a motor the 60km to Phnom Penh, but given River, it's easier to take a short motor ride to the Mekong village of K'am Samnar, where you can get a boat north to Neak Leung (3hr), 37km east of Phnom Penh and connected to the capital by bus and shared taxi.
Note that only Cambodians and Vietnamese are permitted to cross east of Kep, despite assurances to the contrary from Sihanoukvill's Vietnamese consulate.
Cambodia-Vietnam crossing border:
Laos Aviation and Vietnam Airlines operate daily flights from Vientiane to Phnom Penh, with stops in Siem Reap on Tuesday and Fridays; sometimes there's also an unscheduled stop in Pakxe.
Adventurous travelers may wish to try the crossing between Voeng Kham by boat or Dong Khon (Parkse) by land, and the Cambodian town of Stung Treng. Visa can be obtained at Dong Krolar border check point - Cambodia side with feee of 20usd. This will take about 6 to 7 hour to reach Phnom Penh city by bus or taxi.
Cambodia-Lao crossing border:
Updated 14 November 2011