Gentle ensemble refinement is a powerful tool for molecular simulations and modeling. Empirical force fields rely on approximations in their functional form to trade off efficiency and accuracy. Therefore, not all errors in the force field can be resolved just by reparameterization. Also, ensembles generated from other methods than molecular simulations suffer from approximations and benefit from ensemble refinement. The inaccuracies introduced by such approximations can be alleviated using gentle ensemble refinement to integrate system-specific information, most often in the form of experimental data.