We present evidence that nonlinear resonances govern the tunneling process between symmetryrelated islands of regular motion in mixed regular-chaotic systems. In a similar way as for nearintegrable tunneling, such resonances induce couplings between regular states within the islands and states that are supported by the chaotic sea. On the basis of this mechanism, we derive a semiclassical expression for the average tunneling rate, which yields good agreement in comparison with the exact quantum tunneling rates calculated for the kicked rotor and the kicked Harper.PACS numbers: 05.45. Mt, 03.65.Sq, 03.65.Xp Despite its genuine quantal character, dynamical tunneling [1] is strongly sensitive to details of the underlying classical phase space [2]. A particularly prominent scenario in this context is "chaos-assisted" tunneling [3,4,5,6] which takes place between quantum states that are localized on two symmetry-related regular islands in a mixed regular-chaotic phase space. The presence of an appreciable chaotic layer between the islands dramatically enhances the associated tunneling rate as compared to the integrable case, and induces strong fluctuations of the rate at variations of external parameters [3,4]. This phenomenon is attributed to the influence of "chaotic states" that are distributed over the stochastic sea. Since such chaotic states typically exhibit an appreciable overlap with the boundary regions of both islands, they may provide efficient "shortcuts" between the two regular quasimodes in the islands [4,5,6]. Indeed, chaosassisted tunneling processes arise in a number of physical systems, e.g. in the ionization of resonantly driven hydrogen [7], in microwave or optical cavities [8,9], as well as in the effective pendulum dynamics describing tunneling experiments of cold atoms in optical lattices [10,11].While the statistical properties of the chaos-assisted tunneling rates are well reproduced by a random matrix description of the chaotic part of the Hamiltonian [12], the formulation of a tractable and reliable semiclassical theory for the average tunneling rate is still an open problem. Promising progress in this direction was reported by Shudo and coworkers [13] who obtain a good quantitative reproduction of classically forbidden propagation processes in mixed systems by incorporating complex trajectories into the semiclassical propagator Their approach requires, however, the study of highly nontrivial structures in complex phase space, and cannot be straightforwardly connected to single coupling matrix elements between regular and chaotic states. A complementary ansatz, based on a Bardeen-type expression for the coupling to the chaos, was presented by Podolsky and Narimanov [14]. In comparison with tunneling rates from the driven pendulum, good agreement was obtained for large and moderate , whereas significant deviations seem to occur deep in the semiclassical regime [14].In the present Letter, we shall point out that nonlinear resonances between different classical degrees of freedom play a cr...