Conditions for hot isostatic processing (HIP) differ widely from conventional processing, particularly in pressure and atmosphere. Sintering dynamics and thermodynamics in this environment are compared to those in more conventional processes such as hot pressing and normal sintering. It is shown that there are situations in which only HIP can produce certain materials. Intermetallics and nitride ceramics sintered by HIP and capsule HIP, as well as superconductor ceramics processed by oxygen HIP, are reviewed.