General Relativity disbands the Newtonian concept of absolute space,
which gobally determines the inertial properties of matter without
being dynamically back-reacted upon. Rather, according to Einstein's
equations, local inertial systems are dynamically determined by
the gravitational field which itself couples to the distributions
and currents of energy and momentum of matter. A fundamental
consequence of this fact is the so-called "gravitomagnetic field",
which in case of our Earth has just been measured in isolation by the
Gravity-Probe-B experiment (currently being analyzed).
In my talk I will try to convey the essential conceptual ideas
as well as some interesting facts concerning the experiments.