We give an overview about the state-of-the-art in cool stellar (and sub-stellar) atmosphere simulations. Recent developments in numerical methods and parallel supercomputers, as well as in the quality of input data such as atomic and molecular line lists have led to substantial improvements in the quality of synthetic spectra when compared to multi-wavelength observations. A wide range of objects from M dwarfs and giants down to substellar objects is considered. We discuss effects such as atomic and molecular NLTE (and) line blanketing, external irradiation, and formation and opacities of dust particles and clouds; each of which affects the structure of the atmospheres and their spectra. Current models can simultaneously fit many of the observed features of a given star with a single model atmosphere, however, a number of problems remain unsolved and will have to be addressed in the future, in particular for very low mass stars and substellar objects.