  Gravitational vacuum condensate stars, proposed as the endpoint of
gravitational collapse consistent with quantum theory, are reviewed. Gravastars
are cold, low entropy, maximally compact objects characterized by a surface
boundary layer and physical surface tension instead of an event horizon. Within
this thin boundary layer the effective vacuum energy changes rapidly, such that
the interior of a non-rotating gravastar is a non-singular static patch of de
Sitter space with eq. of state p=-rho. Remarkably, essentially this same result
is obtained by extrapolating Schwarzschild's 1916 constant density interior
solution to its compact limit, showing how the black hole singularity theorems
and the Buchdahl compactness bound are evaded. The surface stress tensor on the
horizon is determined by a modification of the Lanczos-Israel junction
conditions for null hypersurfaces, which is applied to rotating gravastar
solutions as well. The fundamental basis for the quantum phase transition at
the horizon is the stress tensor of the conformal anomaly, depending upon a new
light scalar field in the low energy effective action for gravity. This scalar
conformalon field allows the effective value of the vacuum energy described as
a condensate of an exact 4-form abelian gauge field to change at the horizon.
The resulting effective theory thus replaces the fixed constant Lambda of
classical general relativity, and its apparently unnaturally large sensitivity
to UV physics, with a dynamical condensate whose ground state value in empty
flat space is zero identically. This provides both a solution to the
cosmological constant problem and an effective Lagrangian dynamical framework
for the boundary layer and interior of gravitational condensate stars. The
status of present observational constraints and prospects for detection of
gravastars through their gravitational wave and echo signatures are discussed.
