U.S. Citizens originating a Europe trip from Canada

That plan is no problem with just your passport. Seattle people fly from Vancouver all the time.

Passports only long as aren't going to Russia or a county requiring visas.

I have flown out of Montreal to Europe. No problem with that. The biggest hassle (and that wasn't that bad) was getting back into the US from Canada. Much more scrutiny there than when I flew back directly to the US.

We recently had the same experience as the poster immediately above. No big problem, just seemed like somewhat more hoops to jump through when returning at the Canadian airport, but from the US security folks.

In case you don't know what the previous posters are referring to: when you fly from Montreal to the US, you go through US customs and immigration in Montreal. Your arrival in the US is like a domestic arrival, with no immigration or customs. This allows flights from Montreal to airports like New York LaGuardia, which has no international arrival facilities.

Similarly, coming from Europe you will land at the "International" section of Montreal airport, but leaving to the US you depart from the "Transborder" section (not the "Domestic" or "International" section). I haven't looked at the airport maps to see how far apart these are.

As others have mentioned, Montreal (and many other Canadian airports) has U.S. CBP pre-clearance, so you will clear U.S. customs in Montreal if you fly back through Montreal and connect on to another flight to the United States. Just as if you'd clear U.S. customs in the U.S., you'll need to collect your checked baggage, go through customs, recheck your checked baggage, etc. So, best to allow plenty of time for that on the return flight, if you'll be flying on to the United States from Montreal.

However, that's a moot point if you drive to/from Montreal. In that case, you'll clear Canadian customs in Montreal's airport, then clear U.S. customs at the land crossing when you re-enter the United States by car.