Philippine and Malaysian citizens can travel to Cambodia for 21 and 30 days respectively without a visa, but all other nationalities are required to have a visa upon entering Cambodia. Visas are available on arrival at both international airports in Phnom Penh and Siem Reap, and at some land crossings but not all. It is US$20 for a 1 month tourist visa and US$25 for a business visa which is good for 3 months and can be extended multiple times. You will need a passport that is valid for at least the next 6 months and 1 passport photo.
For overland entries, visas are available upon arrival at the Vietnamese (Bavet and Kaam Samnor) and Thai border crossings (Poipet and Krong Koh Kong) but not the Lao crossings (Koh Chheuteal Thom or Dom Kralor). Note that at the Thai crossings at Poipet and Krong Koh Kong, it is 1,000 baht for a tourist visa and 1,500 baht for a business visa, which is higher than the standard rate elsewhere, and border officials may refuse US dollars.
Beware of entry scams: do not be scammed by drivers or others who insist you need their help to get a visa, guards claiming there is a fee for a SARS form or immigration police overcharging for the visas. Particularly for the latter two, it is probably more effective to refuse politely, pretend to not understand or ask for a receipt (which they wouldn't be able to produce for false fees) rather than losing your temper or shouting.
Renew your visa through a Cambodia travel agent or the Department of Immigration on Airport Road (Confederation de Russie) opposite.
We suggest you check this information with your relevant government consulate for the most current information regarding Cambodian Visas and Passport requirements for entry.
US citizens who do not wish to go through the trouble of dealing with the red tape themselves can count on the professional services of Travel Visa Pro to speed things up: Apply for Cambodia Visa Online (service starts from $39). If you need to renew, add pages, change name, or just get a new US passport, you can Apply for a New US Passport here.