  Spectral diagnostic features formed in the solar chromosphere are few and
difficult to interpret -- they are neither formed in the optically thin regime
nor in local thermodynamic equilibrium (LTE). To probe the state of the
chromosphere, both from observations and theory, it is therefore necessary with
modeling. I discuss both traditional semi-empirical modeling, numerical
experiments illustrating important ingredients necessary for a self-consistent
theoretical modeling of the solar chromosphere and the first results of such
models.
