I have not found leaking problems, but I was very disappointed to find that there was no air hole in the top of the cup, making drinking nearly impossible. I solved this by drilling a small hole in the top myself, but we shouldn't have to do this.I find that it is important to fully screw on the lid and check the seal. It can be tricky.The best part: keeps the beverage hot. But no return air hole? Ridiculous.