  In recent years it became clear that a population of galaxies radiating most
of their power in the far-infrared contribute an important part of the whole
star formation activity in the universe. These galaxies emit up to 99% of their
energy output in the infrared by dust. The optical properties of dust in
galaxies are thus very important to understand this population. Mayo Greenberg
who has brought many of the ideas underlying our understanding of the physics
of interstellar dust has been a pioneer in predicting the important role of
organic solid material formed on dust grains in molecular clouds and of
potential importance of transient heating of very small particles. It appeared
with the ISO data on galaxies and even more today with the first Spitzer data
that these mechanisms are important globally for the observations of infrared
galaxies at significant redshifts. The understanding of their evolution is one
of the keys to the understanding of galaxy built up and evolution.
