  With the launch of JWST and the upcoming installation of extremely large
telescopes, the first galaxies in our Universe will finally be revealed. Their
light will be dominated by massive stars, which peak in in the ultra-violet
(UV) part of the electromagnetic spectrum. Star formation is the key driver of
the evolution of our Universe. At young ages, within 10 Million years, both
high and low mass stars generate complex UV emission processes which are poorly
understood yet are vital for interpreting high red-shift line emission. For
these reasons, the Hubble Space Telescope (HST) will devote 1000 orbits to
obtaining a UV Legacy Library of Young Stars as Essential Standards (ULLYSES).
The purpose of this Overview is to outline the basic physical principles
driving UV emission processes from local (within 100 parsecs of) star
formation, ranging from huge star-forming complexes containing hundreds of
massive and very-massive stars (VMS), such as 30 Doradus (the Tarantula Nebula)
in the neighbouring Magellanic Clouds (only 50 kpc away), to galaxies near and
far, out to the epoch of Cosmic Reionization.
