Non-geometric frames in string theory are related to the geometric ones by certain local O(D, D) transformations, the so-called β-transforms. For each such transformation, we show that there exists both a natural field redefinition of the metric and the Kalb-Ramond two-form as well as an associated Lie algebroid. We furthermore prove that the all-order low-energy effective action of the superstring, written in terms of the redefined fields, can be expressed through differential-geometric objects of the corresponding Lie algebroid. Thus, the latter provides a natural framework for effective superstring actions in non-geometric frames. Relations of this new formalism to double field theory and to the description of non-geometric backgrounds such as T-folds are discussed as well.R. Blumenhagen et al.: Non-geometric frames in string theory developed where the O(D, D) transformations 1 play a crucial role, namely generalized geometry [3][4][5][6] and double field theory (DFT) [7][8][9][10][11]. In the first approach, the concept of Riemannian geometry is extended from the tangent bundle T M to the generalized tangent bundle T M ⊕ T * M , whereas in the second the dimension of the space is doubled by including winding coordinates subject to certain constraints. For the latter construction, this admits a manifest global O(D, D) invariance of the action, so in particular, the action is manifestly invariant under T-duality transformations. The fundamental object in both approaches is a generalized metric which combines the usual metric and Kalb-Ramond field. The two local symmetries, diffeomorphisms and B-field gauge transformations, sit inside a subgroup of O (D, D). Their complement in O(D, D) contains so-called (local) β-transforms, which lead out of the usual geometric frame of string theory. Therefore, applying a local β-transform to the geometric frame leads to what we call a non-geometric frame.The existence of non-geometric backgrounds can be seen by analyzing the action of T-duality on the simple background of a flat three-dimensional torus with a constant H-flux [12]. Applying successive T-dualities, this H-flux is first mapped to a geometric flux [13] and by a second T-duality to the nongeometric Q-flux [14][15][16]. The latter background can be understood as a T-fold [17], where the transition functions between two charts involve T-duality transformations. A third T-duality is beyond the scope of the Buscher rules, and both non-commutative geometry [18][19][20] and conformal field theory [21-25] hint towards a non-associative structure. The effect of T-duality on brane solutions has been analyzed recently in [26].Since in DFT a global O(D, D) symmetry is manifest, the first-order effective action in at least a subset of these non-geometric frames is also described by it. What has been puzzling is that the DFT action cannot be straightforwardly interpreted as the Einstein-Hilbert action of some O(D, D) covariant differential geometry [27,28]. The problem is that the notions of torsion and curvature have to be c...