As the largest virialized structures in the universe, galaxy clusters
continue to grow and accrete matter from the cosmic web. Due to the low gas
density in the outskirts of clusters, measurements are very challenging,
requiring extremely sensitive telescopes across the entire electromagnetic
spectrum. Observations using X-rays, the Sunyaev-Zeldovich effect, and weak
lensing and galaxy distributions from the optical band, have over the last
decade helped to unravel this exciting new frontier of cluster astrophysics,
where the infall and virialization of matter takes place. Here, we review the
current state of the art in our observational and theoretical understanding of
cluster outskirts, and discuss future prospects for exploration using newly
planned and proposed observatories.Comment: 56 pages. Review paper. Published in Space Science Review
