If you are arriving into Cambodia at Siem Reap airport, strongly recommend getting an eVisa in advance.. although it is $5 more expensive, it will save you a huge amount of time queuing at the visa on arrival counter. The website is effiicient and took only 2 days to process - well, it could be done in 10 minutes - all you need is a digital passport-style photo & credit card, and access to a printer to print off the visa (black & white accepted). TIP: we found out that for some countries (eg Sri Lanka) the immigration also need some letter of invitation/endoresment from a Cambodian - worth checking up in advance and getting yr hotel or travel agent to send this to you before you leave.  This e-visa thing seems very efficient but is not without risks. Many people have presented their e-visa print-outs at the airport and were told that their names are not on the list (there is a manual list kept at the airport) and were made to apply again and pay another USD20.00. Although you will be given a name and contact number if you wish a refund, but how many would bother.