Dislocations are topological singularities in crystals, which may be described by lines to which a lattice-valued vector, called Burgers vector, is associated. They may be identified with divergence-free matrix-valued measures supported on curves or with 1-currents with multiplicity in a lattice. In the modeling of dislocations one is thus often lead to energies concentrated on lines, where the integrand depends on the orientation and on the Burgers vector of the dislocation. In this talk I will present the theory of relaxation for such energies and I will show how they may arise in a multiscale analysis of dislocations, starting from discrete and semi-discrete models.