  After an introduction on phenomena due to spin and mass-energy currents on
clocks and photons, we review the 1995-2001 measurements of the gravitomagnetic
field of Earth and Lense-Thirring effect obtained by analyzing the orbits of
the two laser-ranged satellites LAGEOS and LAGEOS II; this method has provided
a direct measurement of Earth's gravitomagnetism with accuracy of the order of
20 %. A future accurate measurement of the Lense-Thirring effect, at the level
of 1 % accuracy, may include the LARES experiment that will also provide other
basic tests of general relativity and gravitation. Finally, we report the
latest measurement of the Lense-Thirring effect, obtained in 2002 with the
LAGEOS satellites over nearly 8 years of data. This 2002 result fully confirms
and improves our previous measurements of the Earth frame-dragging: the
Lense-Thirring effect exists and its experimental value is within ~ 20 % of
what is predicted by Einstein's theory of general relativity.
