  X-ray timing observations of neutron stars and black holes are among the few
available probes of ultrastrong magnetic fields, strong gravity, high
densities, and the propagation of thermonuclear burning. Here we review the
evidence for these effects revealed with data from the Rossi Explorer in the
last five years. We also discuss the exciting prospects for making the first
quantitative tests of strong-gravity general relativistic predictions with a
large-area X-ray timing mission.
