When W. Shurcliff published in 1962 the first comprehensive book on polarised light he wisely pointed out: "If light is man's most useful tool, polarised light is the quintessence of utility". The present Euro Summer School on "Astrophysical Spectropolarirnetry" (the first on this topic) comes at a time of greatly increasing interest in this subject. This is mainly due to the new observational possibilities that space telescopes (like the Hubble Space Telescope) and the new generation of large earth-based telescopes are now allowing. Some of these large telescopes (with 8m and 10m diameter mirrors) have recently been installed at some of the best observatories world-wide (e.g. the two Gemini telescopes on Hawai and Chile, the Keck telescopes on Hawai, or the ESO VLT project on Chile), while others are presently under construction, like the Spanish "Gran Telescopio de Canarias" at the Instituto de Astrofisica de Canarias' Roque de los Muchachos Observatory (La Palma, Canary Islands, Spain).
Thanks to the large collecting area of such telescopes, spectro-polarimetry is emerging as an increasingly important tool to study a large variety of astrophysical problems, ranging from Be stars, Wolf-Rayet stars, cataclysmic variables, novae, supernovae, planetary nebulae, and star forming regions to active galactic nuclei, quasars, radio galaxies and the cosmic microwave background itself. This is because spectro-polarimetry provides powerful diagnostics of the physical conditions in astrophysical plasmas (e.g. about magnetic fields) that cannot be obtained via conventional spectroscopy. Although spectro-polarimetry has already led to and will continue to lead to important discoveries in Solar Physics, its application to the remaining fields in Astrophysics is still in its infancy.