What would you do if you find out that your partner lied to you?

Depends, what did he lie about? Like, what level of lying are we talking about here? "No babe, I didn't eat the last debbie cake!" and finding the wrapper on his desk later. Or are we talking, "I don't understand why you're so upset! She just asked about work. I would never do that to you!" and then finding her nudes and plans to meet up with her later on his phone. Be specific.