  Spectra of quasars at high redshift with high resolution and high
signal-to-noise allow in favorable circumstances detection of absorption by
deuterium and ultimately measurement of its primordial abundance. Ultraviolet
spectra of high redshift quasars allow measurement of absorption by the most
abundant cosmic absorber, singly ionized helium, thereby mapping gas even in
the most rarefied cosmic voids. These new techniques already provide
significant constraints on cosmological models but will soon become much more
precise.
