I will be flying from Tennessee to Greece in March. Most of the flights I can book go through Chicago then Toronto to Athens. When coming and going do I go through customs in Canada if I never leave the international section of the airport?
No. You go through customs when returning to the US. You walk theough Nothing to Declare in other countries upon arrival.
DOr, do you mean passport contriol/ immigration?
This link below explains what happens in Toronto
You question is not very clear so I will make some assumptions.
Other than showing your passport to the airline in the US, the US does not have exit passport control. Your are intransit in Toronto so there is no passport or customs if you don't leave the terminal. In Greece you do both passport control (immigration) and customs. On return you will do exit immigration in Greece, and in Toronto you will do passport control for the US prior to the US flight.
I believe some of what has been stated above is inaccurate. If you are returning home through Toronto then you will go through US immigration and customs at Toronto (YYZ) since it is a preclearance facility. The Toronto - US flight is always treated as a domestic one with no passport control on US soil
It is possible I misunderstood your question. If so please restate