The first specifically Cambodian stamps were produced in 1952.
The Kingdom wasn't genuinely independent at the time, but its first sheets of stamps were produced, in a break from the 'Indo-China' marked ones used throughout the region under French colonial rule.
There have been stamps released since under a variety of different categorisations, as Cambodia's regimes have changed frequently.
Stamp categories under different kings include King Norodom Sihanouk's Royaume du Cambodge and Lon Nol's Republique Khmere. King Norodom Sihanouk's reign is particularly well illustrated through stamps.
Supposedly, there are also stamps from Pol Pot's time, but these have passed into myth.
The Communist revolutionaries, Khmer Rouge banned telecommunications, but in April 1978 a German catalogue suggested that some had been printed under the Democratic Kampuchea designation. These would most likely have been used for propaganda.
Any stamps fitting that description would be immensely desirable to philatelists.
Immediately after Pol Pot was toppled, the government needed stamps and solved the problem by overprinting pre-Khmer Rouge ones with RPK (Republique Populaire Kampuchea). These overprints would also be valuable.
Later 1990s stamps with added surcharges are also of great interest to collectors, but unfortunately they have proved relatively easy to forge. The surcharges were added because inflation had overtaken the rate at which stamps could be printed.
There are some 1990s stamps which have safer, if not spectacular, values. A set of five space travel-themed stamps issued in 1990 can fetch $1,100.
Overall, Cambodia offers an excellent example of what philately is all about: rare pieces which illustrate events in the history of countries and the world.