In this talk I will be discussing some reformulations of string theory which promote T-duality to the level of a manifest symmetry namely Hull's Doubled Formalism and Klimcik and Severa's Poisson-Lie T-duality. Such formalisms double the number of fields but also incorporate some chirality-like constraint. Invoking this constraint leads one to consider sigma-models which, though duality invariant, do not possess manifest Lorentz Invariance. Whilst such formalisms make sense at a classical level their quantum validity is less obvious. I address this issue by examining the renormalization of these duality invariant sigma models. This talk is based upon both forthcoming work and recent work in arXiv:0910.1345 [hep-th] and its antecedents arXiv:0708.2267, arXiv:0712.1121.
