The extinction caused by the diffuse interstellar medium (ISM) has been extensively studied in the wavelength range from UV to near-IR. The dust, the cause of this extinction, its physics and chemistry, its origin and its environments are well described. During the latest decade satellites (e g ISO, Spitzer) have provided us with the tools to extend the wavelength range to mid-IR and far-IR. This opens the possibility to investigate even the dense parts of the ISM, the molecular clouds. We have studied the small globule Barnard 335 from UV to mid-IR. The different techniques to measure the extinction will be the sub ject of the seminar as well as the results of our study.
