Observational evidence for dust growth in proto-planetary discs

The dust in the interstellar medium, that provides the material for forming stars - and circumstellar discs as a natural by-product - is known to have submicron sizes. As these discs are the sites of planet formation, those small grains are predicted to grow to larger entities when the stars are still young. will review evidence for the first steps in grain growth in proto-planetary discs around young stars, based on recent Spitzer and ground-based infrared observations. First, I will discuss disc and dust properties in Herbig Ae/Be stars, and then move to the lower-mass T Tauri stars and the brown dwarfs. Here, objects of different star-forming regions are compared, and the influence of the stellar parameters and environment on dust evolution, as witnessed by the observed dust characteristics, is discussed.