You go through US customs and immigration at Toronto Pearson Airport so you don't have to worry about it when you get to Chicago (i've flown the Toronto - Chicago route before), but even still 45 minutes is too short of a layover, you need at least an hour due to a number of reasons like your flight being delayed, arriving late, getting from one gate to another (Chicago is a big airport) etc.
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^ Yes.Originally posted by Riyko View Post
Use maps to figure out where your flight is ahead of time. I had a short layover at Chicago and was sitting in the wrong terminal before I realised "doh!" and I made it as they were starting boarding. Even if you can't put the map from the paper into use at the actual airport, it's at least swirling around inside your head somewhere.
And pre-clearance is so nice. Knowing you don't have to go through customs, immigration, etc... Yeah, if your flight is on time, and this is assuming it is, you'll make it, no problem. 
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