  In this contribution we discuss about numerical modeling of nebulae. In
particular we emphasize on the dynamical evolution of an HII region and the
chemical structure of a Photodissociation region. We do this using the Smoothed
Particles Hydrodynamics code SEREN and the recently developed astrochemistry
code 3D-PDR, respectively. We show an example application by simulating a
cometary globule using these two codes.
