As with most countries in the region, you will be asked to fill out an arrival card before entering Cambodia, whether at Angkor International Airport or any other immigration and customs point into the country. This form will ask various questions about your status and purpose of visit, and demand you declare any illegal items or cash over a certain amount whether in riel or any other currency.
Illegal materials in Cambodia follow a pattern similar to any other country. Narcotics including marijuana are illegal as are firearms and pornography.
One litre of alcohol, 200 cigarettes (or 50 cigars) or up to 250g of tobacco is permitted through Cambodian customs. Please refrain from trying to chip a piece of Angkor Wat off to take home with you or indeed from buying anything that is purported to have been removed from Angkor. Cambodian customs are very strict about such items and some countries, including the US, have passed laws designed to discourage the plundering of the site since its popularity has escalated in the past decade