Originally Posted by TravellingChris

If you're coming from Brussels, you will clear U.S. Customs and Border Protection in Toronto but will not have to undergo Canadian customs. You should not have to retrieve your bags which saves a ton of time. However, the wait at U.S. preclearance will depend on passenger volume and time of day. You will have to undergo security screening at YYZ as the U.S. does not recognize EU aviation security (Canada does).