I'm a Canadian citizen and will be travelling in Cambodia for two weeks with plans to visit Laos.
Do I need a visa to pass from Cambodia into Laos and then back into Cambodia?
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For Laos, Canadian citizens require a Single Entry visa, valid for up to 60 days.
Cambodia you require a single entry visa, valid for 90 days.
However, in both cases, while they're valid for 60 and 90 days respectively, this is just the period after application during which you enter, and you can only actually stay in each country for 30 days, or when you leave, whichever is sooner. So if you leave Cambodia to Laos, you'll need ANOTHER Cambodian visa to return.
The official price for a tourist visa for Cambodia is US$20, and US$25 for an Ordinary visa - but expect much higher prices (US$30 or more for the tourist visa) to be demanded at land border crossings. Visas can be obtained at any Cambodian embassy or consulate overseas. Visas are also available "on arrival" at both international airports, all six international border crossings with Thailand, some international border crossings with Vietnam, and at the main border crossing with Laos.
Now we start getting clever. Instead of a tourist visa, you could consider a Type-E or Ordinary visa, as you can configure them for multiple entries, meaning you won't have to apply for two separate visas for Cambodia.
For Laos, most countries can get a visa on arrival at the airport, but not sure about land crossings. Only price I can find there is $42 for Canadians according to Wikitravel.
Hopefully some of that info and the links are useful!
Mark's answer covers most parts so I'll throw in a couple of notes from my experience:
Now for the Cambodia to Laos bit. I asked a question about this earlier here. If you want the easy way out you can look around for travel agencies which will arrange this for you. I wanted to do it DIY. Although Siem Reap is technically closer to Laos then Phnom Penh is, I didn't find any bus operators operating buses directly at the time. The closest border town is Stung Treng but for some reason it's easier to find buses to this town from Phnom Penh that it is from Siem Reap. Once at Stung Treng, you need to go on further on unorganised public transport.
Although visa-on-arrival is technically available for Laos at border checkpoints, when I visited this year a couple of months ago I was advised by the owner of a hostel I stayed at in Phnom Penh that they were no longer issuing them at the border and you'd need to get one in advance. I'm not sure whether this was actually true or still is, and I don't think it was a scam because he didn't direct any of us to any travel agencies to 'process' the visa anyway - he gave contact details for the Lao Embassy in Cambodia. Apparently it doesn't take more than a day or two to get a Lao visa in Cambodia itself.
Take a call on whether you want to get it before you begin travel, or get to Cambodia, find out whether they are still issuing at the border... and if not then get it in Phnom Penh.