By Air: Regular air service links Phnom Penh and Siem Reap to Bangkok and Vientiane. Domestic flights run between Phnom Penh and Siem Reap. Air Asia, Bangkok Airways, Lao Airlines, Royal Khmer Airlines, Siem Reap Airways, and Thai Airways have flights to Cambodia.
By Boat: From Phnom Penh, ferries called "bullet boats" travel along the Tonle Sap to reach Siem Reap and Angkor; they also ply waters between Sihanoukville and Koh Kong. You can buy tickets from a tour operator, your hotel's concierge, or at the port in Phnom Penh. Bullet boats, though fast, can be dangerous. Smaller ferries travel daily between Siem Reap's port and Battambang, on the Sangker River. Ask about water levels before booking a ticket; in dry season, the water can get so low the boat may get stuck for hours at a time.
Compagnie Fluviale Du Mekong (30 St. 240, Phnom Penh. 023/216070. www.cfmekong.com) offers three-day luxury river excursions between Phnom Penh and Siem Reap. Boats depart Phnom Penh from the municipal port on Sisowath Quay, at Street 84, early in the morning (about $26 one way).
By Bus: Cambodia has a comprehensive bus network, and bus travel is cheap. It's also generally the safest cross-country transportation, aside from flying. Travel from neighboring countries is easy, reliable, and cheap. Buses from Thailand and Vietnam operate daily.