Customs Question

Here's are US Customs pre-clearance locations:
http://www.cbp.gov/border-security/ports-entry/operations/preclearance

Through preclearance, the same immigration, customs, and agriculture
inspections of international air passengers performed on arrival in
the United States can be completed before departure at foreign
airports instead. Currently, preclearance operations take place at 15
foreign airports in six different countries, benefitting air
passengers, airports, and air carriers, in the United States and
abroad.

Canada is "different." Check carefully. I don't know if this is currently in effect, but not so long ago, flying through Toronto required going through passport control and customs even for transit passengers. You would still have to go through passport control and customs in Europe.

Since Montreal has preclearance procedures, assume you go through passport control and customs there. The plane would then arrive as a domestic flight at O'Hare.

My guess is that you'll have to collect your checked bags before clearing customs, then recheck them for your next flight. As you are considered to be entering Canada, you go through passport control when you land, collect your bags, go through Canadian customs, then check bags to Venice and passport control and security before going to your gate. Coming back, add going through U.S. customs before going to your gate for the flight to Chicago.

To be precise, "customs" refers only to your belongings. The immigration and security procedures come first, and will be handled in Monreal by agents from your own country. Be sure to get in the right line, as Canadians and foreigners staying in Canada go through a different procedure with Canadian officials. I don't know if your luggage will be inspected too; the flight to Chicago qualifies as international so you may only need to retrieve your luggage for customs there. Your airline probably knows.

Since Montreal apparently is a US preclearance airport, it may be that the same procedures are followed as at other preclearance locations. When we flew back to the US from mainland Europe via Dublin, we never had to touch our checked baggage. As we were going through US immigration there, the agent pulled up our baggage on his screen and asked us if that was ours. I don't know where the bags were.

Security, customs, and immigration are three different things, even though many USers say "customs" to mean all three. Don't worry about going to the "wrong" place - they won't let you skip any necessary steps.

When Chani said that Canada is "different," what she means is that major Canadian airports (including Toronto and Montreal) have US Customs and Border Patrol staff. If you were just flying from Canada to the US, you go through "preclearance" in Canada, and your landing in the US is like a domestic flight. These flights are called "transborder" and are in a different section of the airport than either "domestic" (all in Canada) or "international" (destinations besides the US and Canada).

Until recently, if you were flying from Europe through Canada to the US, you had to "enter Canada" then "leave Canada" and "enter the US" all in the Canadian airport. However, recently the airports were reconfigured to allow more US passengers to transit through Canada without going through Canadian immigration and customs (again, you will still go through US formalities in Montreal). Whether you can do this or not depends on the specifics of your flight (airlines, destinations, etc).

Here is the page for Toronto Airport; put in your specific flights and it will have the details: http://www.torontopearson.com/Connecting.aspx#

Here's the information for Montreal Airport: http://www.admtl.com/en/guide/connecting-flights

Glad to hear Canada has become even more friendly to its southern neighbors.

I flew from Calgary - a preclearance airport - to O'Hare about 2.5 years ago and we landed at a domestic terminal even though it's technically an international flight. That appears to be the situation from the Montreal airport link Harold provided:

If your flight originated outside Canada or the United States and you need to take a connecting flight to a U.S. destination, there are two possible options:
If your airline *offers** the option of automatically transferring your luggage and you have a boarding pass for your connecting flight, you will be asked to proceed via a connecting flights corridor, avoiding Canadian customs. You will, however, have to go through U.S. customs. Your checked luggage will be transferred automatically by the luggage-handling system. Note, however, that U.S. customs officers are authorized to recall your luggage, if necessary, for manual inspection. If the customs officers do not yet have the information on your luggage at hand, you will be asked to proceed to the waiting area.
If your airline does not offer the option of automatically transferring your luggage, you must pick it up yourself from the carousel, then follow the normal procedures for international arrivals, then for U.S. departures.*

So if your airline has automatic transfer, you cruise through. If not, allow time to pick up your luggage (after Canadian passport control), go through Canadian customs and continue to U.S. pre-screening, including rechecking your luggage somewhere along the line.

Toronto is likely to have the same procedures. Again it depends on whether your airline has the automatic transfer option.

On the Toronto site, you put in your airline/s and it tells you the steps you take. You can probably use it to see the exact steps for Montreal too. If you are flying on different carriers into and out of Canada (even though your ticket is issued by one airline), be sure to put in the specific carrier for each leg.

All the major Canadian airports have pre clearance for your return trip back to the US (Toronto, Montreal, Vancouver, Calgary, and so on). This means when you get off your international flight into the Canadian airport, you will go through US customs and immigration administered by US personnel and then enter the US wing of the airport. You cannot exit that wing except to board your flight into the US.

On the way to Europe passing through Canada you do need to fill out a Canadian immigration document (or at least I did last time though 6 months ago). But you walk through a simple process where you hand in the paper and your passport is checked and then you go to the international wing of the airport. Once again, you are not allowed to exit that wing into Canada proper.

If you have a long layover in Canada or will be spending the night before your flight out, you have the option of going through the complete Canadian customs and immigration process and officially entering Canada.