The Berger conjecture states that if M is a simply connected, complete Riemannian manifold all of whose geodesics are closed, then all geodesics have the same least period. Not much is know about this conjecture, apart from dimension two, where it is a theorem by Grove and Gromoll. I will present a proof of this theorem and maybe describe some nonstandard metrics on the two sphere with all geodesics closed of the same least period.