From The Avengers: Age of Ultron
[from preview footage
[None of the Avengers can lift Mjolnir
It's biometrics, right? Like a security code? "Whoever is carrying Thor's fingerprints" is, I think, the literal translation.
Yes, well that's a very, very interesting theory. I have a simpler one: You are not worthy.
[an attack occurs
] Worthy? How could you be worthy? You're all killers. You want to protect the world, but you don't want it to change. There's only one path to peace... your extinction.