Two inverse problems for the Sturm-Liouville operator Ly = −y + q(x)y on the interval [0, π] are studied. For θ 0, there is a mapping F : W θ 2 → l θ B , F (σ) = {s k } ∞ 1 , related to the first of these problems, where W θ 2 = W θ 2 [0, π] is the Sobolev space, σ = q is a primitive of the potential q, and l θ B is a specially constructed finite-dimensional extension of the weighted space l θ 2 , where we place the regularized spectral data s = {s k } ∞ 1 in the problem of reconstruction from two spectra. The main result is uniform lower and upper bounds for σ − σ 1 θ via the l θ B -norm s − s 1 θ of the difference of regularized spectral data. A similar result is obtained for the second inverse problem, that is, the problem of reconstructing the potential from the spectral function of the operator L generated by the Dirichlet boundary conditions. The result is new even for the classical case q ∈ L 2 , which corresponds to θ = 1.Key words: Inverse Sturm-Liouville problem, singular potentials, stability for inverse problems.In the present paper we study two classical inverse problems for the Sturm-Liouville operatoron a finite interval. The first is the problem of reconstructing the potential from the two spectra of the operator (0.1) with Dirichlet and Dirichlet-Neumann boundary conditions, respectively. (We refer to it as Borg's problem.) The second is the problem of reconstructing the potential from the spectral function of the operator (0.1) with Dirichlet boundary conditions. (This operator will be called the Dirichlet operator.) The solution of these problems has long been known for the case of real potentials q ∈ L 2 ; in particular, a complete characterization of spectral data for potentials q of this class has been obtained. Our aim is to solve these problems for potentials q in the scale of Sobolev spaces W α 2 for any given α −1, including the case of α ∈ [−1, 0), where the potential is a singular function (a distribution). Here an important role is played by special Hilbert spaces that we construct to solve these problems. These spaces are needed to define and study mappings which we associate with these problems, as well as to completely describe (characterize) spectral data for potentials with primitive σ = q(t) dt ranging over the set of real functions in W α+1 2 .Once the inverse problems are solved, there arises an important question about a priori estimates, namely, the question of how small the change in a primitive of the potential q is in the norm of W α+1 2 when the spectral data undergo a change small in the norm of the corresponding Hilbert space, in which these data are placed. Earlier, a priori estimates have been obtained in the classical case (for α = 0). But these are estimates of local type, in which the constants, as well as the radius of the neighborhood where the estimates hold, depend on the potential q. The main goal of the present paper is to obtain uniform two-sided a priori estimates not only for the classical case α = 0 but also for all α > −1. The case α = −1 is exceptional...