  We report the observation of radar echoes from the ionization trails of
high-energy particle cascades. These data were taken at the SLAC National
Accelerator Laboratory, where the full electron beam ($\sim$10$^9$ e$^-$ at
$\sim$10 GeV/e$^-$) was directed into a plastic target to simulate an ultra
high-energy neutrino interaction. This target was interrogated with radio
waves, and coherent radio reflections from the cascades were detected, with
properties consistent with theoretical expectations. This is the first
definitive observation of radar echoes from high-energy particle cascades,
which may lead to a viable neutrino detection technology for energies $\gtrsim
10^{16}$ eV.
