The function of flexible biological macromolecules, such as proteins, is tightly linked to their dynamics. NMR spectroscopy proves to be an efficient method to capture the dynamics of biomolecules on time scales of pico-to nanoseconds. In NMR experiments, the relaxation rates are governed by the dynamics of the bonds due to overall and internal motion. As an ideal complement, molecular dynamics (MD) simulations are a powerful tool to understand protein dynamics and how they are linked to thermodynamic quantities. The ability of MD simulations to predict the dynamics of the biomolecules critically depends on the conformational space explored within the simulation time (sampling) and the accuracy of the potential energy function (force field). The common time scale of the motions probed by both methods makes NMR relaxation a key approach to validate MD simulations, while simulation can provide detailed microscopic insights into the structural dynamics that underlie key properties of proteins, including entropy and heat capacity.