We find that general relativity has held up under extensive
experimental scrutiny. The question then arises, why bother to
continue to test it? One reason is that gravity is a fundamental
interaction of nature, and as such requires the most solid
empirical underpinning we can provide. Another is that all
attempts to quantize gravity and to unify it with the other
forces suggest that the standard general relativity of Einstein
is not likely to be the last word. Furthermore, the predictions
of general relativity are fixed; the theory contains no
adjustable constants so nothing can be changed. Thus every test
of the theory is either a potentially deadly test or a possible
probe for new physics. Although it is remarkable that this
theory, born 80 years ago out of almost pure thought, has managed
to survive every test, the possibility of finding a discrepancy
will continue to drive experiments for years to come.