This thesis studies variational problems invariant under a Lie group transformation, and invariant discretizations of these. In chapters two and three, a general method for creating symplectic integrators preserving certain classes of variational symmetries of first order Lagrangians is developed and demonstrated. In chapters four and five, it is assumed that the discrete Lagrangian is invariant under a certain group action, and the Euler--Lagrange equations for the variational problem are expressed in the invariants of the group action.