  Modern formulations of equivalence principles provide the foundation for an
efficient approach to understanding and organizing the structural features of
gravitation field theories. Since theories' predictions reflect differences in
their structures, principles of equivalence also support an efficient
experimental strategy for testing gravitation theories and for exploring the
range of conceivable gravitation physics. These principles focus attention
squarely on empirical consequences of the fundamental structural differences
that distinguish one gravitation theory from another. Interestingly, the
variety of such consequences makes it possible to design and perform
experiments that test equivalence principles stringently but do so in markedly
different ways than the most familiar experimental tests.
