The hydrogen Lyman-alpha line is of utmost importance to many fields of
astrophysics. This UV line being conveniently redshifted with distance
to the visible and even near infrared wavelength ranges, it is
observable from the ground, and provides the main observational window
on the formation and evolution of high redshift galaxies. Absorbing
systems that would otherwise go unnoticed are revealed through the
Lyman-alpha forest, Lyman-limit, and damped Lyman-alpha systems, tracing
the distribution of baryonic matter on large scales, and its chemical
enrichment.
We are living an exciting epoch with the advent of new instruments and
facilities, on board of satellites and on the ground. Wide field and
very sensitive integral field spectrographs are becoming available on
the ground, such as MUSE at the ESO VLT. The giant E-ELT and TMT
telescopes will foster a quantum leap in sensitivity and both spatial
and spectroscopic resolution, to the point of being able, perhaps, to
measure directly the acceleration of the Hubble flow. In space, the JWST
will open new possibilities to study the Lyman-alpha emission of
primordial galaxies in the near infrared. As long as the Hubble Space
Telescope will remain available, the UV-restframe properties of nearby
galaxies will be accessible to our knowledge. Therefore, this Saas-Fee
course appears very timely and should meet the interest of many young
researchers.