re: #297 Dizzy

I don’t believe that is true for Canada. In Canadian airports, US border control maintains a checkpoint that travelers must pass through before boarding.

re: #295 Nerdy Fish

You do not have to clear US customs to get on a US-bound flight from another country. Customs is done when you are exiting the international terminal; if you are using a US airport as a connecting destination between two international flights, you actually never have to traverse US customs at all. As for letting you board a flight, that is entirely up to the security in the country of origin; US customs checks your passport or visa status on the way in, which is where the inadmissible immigrants would get nabbed.

Actually….

US CBP runs what are called Immigration Pre-clearance in a number of international airports and seaports, some in Canada, but also in the main airport of Ireland (Shannon?), the Bahamas, and Abu Dhabi in the UAE.
(I’ve been through the last one.) Not every airport in Canada has them.

Basically, you get your passport (and visas if you need them) checked go through customs, by CBP agents, and then your flight lands in a domestic terminal in the US. The Benefits are that the plane can land at a domestic terminal or a domestic airport without a CBP presence.
The drawbacks are that you have no recourse if you get turned away (even with a valid visa) when not on US soil. (The US has prevented numerous foreign nationals and at least one US citizen from boarding flights to the US.)

Also, for pretty much any flight to the US, every one gets questioned in the boarding area while outside the US (or has been for at least the past 20 years) — many international airports have a closed off area for flights to the US. This happens everywhere. The people are not CBP employees from what I’ve seen.

And if you are connecting between two international flights in the US, you do have to clear immigration at least (not 100% sure about customs). Being an American, or having a valid visa or status to be in the US at least temporarily is required.

Note that Customs and immigration checks are independent of each other. Usually going through one means you have to go through both, but there are parts of the US which are in a different customs zone (like the US Virgin Islands), which means you need to go through Customs without going through passport control.