The study of star formation and of the interstellar medium gained large momentum during the last years, thanks to the immense amount of radio, infrared and far-infrared data observed with the latest instruments on Spitzer, Herschel, ALMA and SOFIA. The interaction of massive star formation and its ambient medium, e.g. the heating of the dust and gas by strong UV radiation, gives rise to Photodissociation Regions (PDRs), an interface region between the ionized gas in HII regions and the cold molecular gas. PDR-models solve the complex problems of astrochemistry, thermodynamics and radiative transfer and provide observable properties that can be compared with observations.

The Cologne PDR-code KOSMA-tau models spherical PDRs including the local gas-phase and surface chemistry, the line and continuum radiative transfer including a variety of possible dust compositions, and the local heating and cooling balance to approximate the chemical and physical structure of the observed PDRs. The spherical clouds can be combined to form a clumpy ensemble of PDRs: The recent development of KOSMA-tau-3D allows to model arbitrary 3-dimensional PDR structures ranging from local star forming regions", e.g. Orion (Andree-Labsch et al. 2017), to whole galaxies, e.g. the Milky Way (Cubick et al. 2007, Bruckmann et al.).

In this talk I will give an overview of the recent code developments, e.g. the full surface chemistry update and the self-consistent continuum radiative transfer and dust modelling including low-metallicity environments such as the Small and Large Magellanic Clouds. I will also present a number of recent applications of the KOSMA-tau results to new data.