  We survey the role of stable clocks in general relativity. Clock comparisons
have provided important tests of the Einstein Equivalence Principle, which
underlies metric gravity. These include tests of the isotropy of clock
comparisons (verification of local Lorentz invariance) and tests of the
homogeneity of clock comparisons (verification of local position invariance).
Comparisons of atomic clocks with gravitational clocks test the Strong
Equivalence Principle by bounding cosmological variations in Newton's constant.
Stable clocks also play a role in the search for gravitational radiation:
comparision of atomic clocks with the binary pulsar's orbital clock has
verified gravitational-wave damping, and phase-sensitive detection of waves
from inspiralling compact binaries using laser interferometric gravitational
observatories will facilitate extraction of useful source information from the
data. Stable clocks together with general relativity have found important
practical applications in navigational systems such as GPS.
