  We highlight the importance of the MgII emission-line doublet in probing
high-redshift quasars and their supermassive black holes. In the SDSS era,
where large scale investigations of quasars across the age of the Universe are
possible, this emission-line has the ability to provide accurate systemic
redshifts which are important for a variety of follow-up studies, as well as
probe the masses of the supermassive black holes that power these phenomena.
