  Isolating ground-based interferometric gravitational wave observatories from
environmental disturbances is one of the great challenges of the advanced
detector era. In order to directly observe gravitational waves, the detector
components and test masses must be highly inertially decoupled from the ground
motion not only to sense the faint strain of space-time induced by
gravitational waves, but also to maintain the resonance of the very sensitive 4
km interferometers.
  This article presents the seismic isolation instrumentation and strategy
developed for Advanced LIGO interferometers. It reviews over a decade of
research on active isolation in the context of gravitational wave detection,
and presents the performance recently achieved with the Advanced LIGO
observatory. Lastly, it discusses prospects for future developments in active
seismic isolation and the anticipated benefits to astrophysical gravitational
wave searches.
  Beyond gravitational wave research, the goal of this article is to provide
detailed isolation strategy guidelines for sensitive ground-based physics
experiments that may benefit from similar levels of inertial isolation.
