The interstellar medium is the counterpart of the stars in the cycle of matter evolution in the Universe. New stars are build from giant gaseous clouds and dead stars are giving back to the interstellar medium new elements synthesized during their life. Understanding the physics of such gaseous nebulae is a key to our knowledge of the chemical evolution of our Universe. I will present in this talk the computational tools used to model such gaseous nebulae, especially the 3D photoionization codes, and some applications to HII regions and Planetary Nebulae.