  Two techniques for computing black hole entropy in generally covariant
gravity theories including arbitrary higher derivative interactions are
studied. The techniques are Wald's Noether charge approach introduced recently,
and a field redefinition method developed in this paper. Wald's results are
extended by establishing that his local geometric expression for the black hole
entropy gives the same result when evaluated on an arbitrary cross-section of a
Killing horizon (rather than just the bifurcation surface). Further, we show
that his expression for the entropy is not affected by ambiguities which arise
in the Noether construction. Using the Noether charge expression, the entropy
is evaluated explicitly for black holes in a wide class of generally covariant
theories. Further, it is shown that the Killing horizon and surface gravity of
a stationary black hole metric are invariant under field redefinitions of the
metric of the form $\bar{g}_{ab}\equiv g_{ab} + \Delta_{ab}$, where
$\Delta_{ab}$ is a tensor field constructed out of stationary fields. Using
this result, a technique is developed for evaluating the black hole entropy in
a given theory in terms of that of another theory related by field
redefinitions. Remarkably, it is established that certain perturbative, first
order, results obtained with this method are in fact {\it exact}. The possible
significance of these results for the problem of finding the statistical origin
of black hole entropy is discussed.}
