  Until recently, only about 10% of the total intracluster gas volume had been
studied with high accuracy, leaving a vast region essentially unexplored. This
is now changing and a wide area of hot gas physics and chemistry awaits
discovery in galaxy cluster outskirts. Also, robust large-scale total mass
profiles and maps are within reach. First observational and theoretical results
in this emerging field have been achieved in recent years with sometimes
surprising findings. Here, we summarize and illustrate the relevant underlying
physical and chemical processes and review the recent progress in X-ray,
Sunyaev--Zel'dovich, and weak gravitational lensing observations of cluster
outskirts, including also brief discussions of technical challenges and
possible future improvements.
