  Line-Intensity Mapping is an emerging technique which promises new insights
into the evolution of the Universe, from star formation at low redshifts to the
epoch of reionization and cosmic dawn. It measures the integrated emission of
atomic and molecular spectral lines from galaxies and the intergalactic medium
over a broad range of frequencies, using instruments with aperture requirements
that are greatly relaxed relative to surveys for single objects. A coordinated,
comprehensive, multi-line intensity-mapping experimental effort can efficiently
probe over 80% of the volume of the observable Universe - a feat beyond the
reach of other methods. Line-intensity mapping will uniquely address a wide
array of pressing mysteries in galaxy evolution, cosmology, and fundamental
physics. Among them are the cosmic history of star formation and galaxy
evolution, the compositions of the interstellar and intergalactic media, the
physical processes that take place during the epoch of reionization,
cosmological inflation, the validity of Einstein's gravity theory on the
largest scales, the nature of dark energy and the origin of dark matter.
