The behaviour of flux line lattices (FLL) in type-II superconductors are heavily affected by the presence of impurities in the sample. Impurities or crystalline defects reduce the order parameter and thereby act as pinning sites for the vortices. At low fields and temperatures the flux line lattice exists in the so-called Bragg glass (BrG) phase, a phase characterized by an algebraically decaying translational order, where the vortices are only weakly distorted by the pinning sites. As the field or temperature is increased dislocations in the flux line lattice cause the quasi-long range order to be replaced by short-range correlations, signifying the transition to a disordered vortex glass phase.