With us. We were each allowed one cat (under the seat in front of us) and one additional personal item (in the overhead bin). I had a backpack, and hubby had a computer bag.

If you have to travel with a pet, I highly recommend the carriers from Mr. Peanut's. (The name of a shelter dog that inspired the company, not the Planters mascot.) Lightweight, easy to clean (SO very important with the one who insists on peeing, pooping, and throwing up in her carrier), sturdy, and sized precisely to fit underneath standard airline seats. Chewy had them on sale at a deep discount of $30 per carrier.