TITAN is a computer program for calculating the interactions of a dilute plane-parallel medium with electromagnetic radiation. It includes all atomic processes: absorption, recombination, diffusion, excitation, deexcitation of atoms and ions, heating and cooling of the gas, and it solves the radiation transfer, in order to obtain the spectra reemitted by the medium. It handles plan parallel slabs in non LTE steady state, for various physical conditions and various illuminations, valid in many astrophysical situations. It is specifically designed for warm-hot (8000 to 10**8 K) and thick media (till an electron scattering optical depth of several tens) emitting and absorbing in the X-ray range (density from 10**5 to 10**14 cm-3). It computes the physical parameters, ionisation degrees, temperature, density, and the spectrum of the radiated light in each point of the slab, by solving simultaneously the ionisation equations, the equations of statistical equilibrium, the thermal equations and the radiation transfer, using iteration processes.
