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Traditionally, the presence of dust in galaxies has been regarded as a nuisance preventing a clear view of stars in galaxies. This is even more true if one is interested in newly formed stars, where dust is not merely a nuisance but a show stopper. This is not only due to the enhanced absorption probability of UV photons radiated predominantly by the young stars, but also because of the strong spatial correlations between young stars and gas and dust in galaxies. Fortunately infrared space astronomy is allowing us in a real sense to hunt for the dark and uncover the obscured star formation, since the stellar photons that are obscured by dust become visible in the infrared (IR). And in fact half of the energy emitted by all stars in the Universe since the Big Bang is absorbed by dust grains and is re-emitted in the IR, as revealed by measurements of the extragalactic background. So understanding dust emission is crucial to the understanding of the star-formation history of the Universe, which is intimately connected to the understanding of the conversion of gas into stars in galaxies over cosmic time.

At the same time dust can influence structure formation in the Universe through even more direct physical mechanisms affecting the thermodynamic state of gas inside and outside of galaxies, since dust is a primary coolant for the gas at all scales, in the intergalactic medium (IGM), in the interstellar medium (ISM) and in the star-forming clouds.

Here we shall review models for the conversion of stellar photons into infrared emission as well as recent progress in our understanding of dust as a main coolant for the gas. The first part is devoted to the mechanisms through which dust can affect the thermodynamic state of the gas from Mpc to pc scales. The second part describes recent progress in our ability to quantitatively model measurements of the amplitude and colours of dust emission to recover the nature of the stellar populations powering the infrared emission and other physical characterics of galaxies.

Previous reviews given on this topic can be found in Popescu & Tuffs (2009), Popescu & Tuffs (2008), Popescu& Tuffs (2007), Popescu& Tuffs (2005), Dopita (2005), Madden (2005), Bianchi (2004).