Abstract. A class of trust-region methods is presented for solving unconstrained nonlinear and possibly nonconvex discretized optimization problems, like those arising in systems governed by partial differential equations. The algorithms in this class make use of the discretization level as a mean of speeding up the computation of the step. This use is recursive, leading to true multilevel/multiscale optimization methods reminiscent of multigrid methods in linear algebra and the solution of partial-differential equations. A simple algorithm of the class is then described and its numerical performance is shown to be numerically promising. This observation then motivates a proof of global convergence to first-order stationary points on the fine grid that is valid for all algorithms in the class.Keywords: nonlinear optimization, multiscale problems, simplified models, recursive algorithms, convergence theory.AMS subject classifications. 90C30, 65K05, 90C26, 90C061. Introduction. Large-scale finite-dimensional optimization problems often arise from the discretization of infinite-dimensional problems, a primary example being optimal-control problems defined in terms of either ordinary or partial differential equations. While the direct solution of such problems for a discretization level is often possible using existing packages for large-scale numerical optimization, this technique typically does make very little use of the fact that the underlying infinite-dimensional problem may be described at several discretization levels; the approach thus rapidly becomes cumbersome. Motivated by this observation, we explore here a class of algorithms which makes explicit use of this fact in the hope of improving efficiency and, possibly, enhancing reliability.Using the different levels of discretization for an infinite-dimensional problem is not a new idea. A simple first approach is to use coarser grids in order to compute approximate solutions which can then be used as starting points for the optimization problem on a finer grid (see [5,6,7,22], for instance). However, potentially more efficient techniques are inspired from the multigrid paradigm in the solution of partial differential equations and associated systems of linear algebraic equations (see, for example, [10,11,12,23,40,42], for descriptions and references). The work presented here was in particular motivated by the paper by Gelman and Mandel [16], the "generalized truncated Newton algorithm" presented in Fisher [15], a talk by Moré [27] and the contributions by Nash and co-authors [25,26,29]. These latter three papers present the description of MG/OPT, a linesearch-based recursive algorithm, an outline of its convergence properties and impressive numerical results. The generalized truncated Newton algorithm and MG/OPT are very similar and, like many linesearch methods, naturally suited to convex problems, although their generalization to the nonconvex case is possible. Further motivation is also provided by the computational success of the low/high-fidelity mo...