  We review recent measurements of the high-frequency dynamic magnetic
susceptibility in the high-$T_c$ superconducting systems
La$_{2-x}$Sr$_{x}$CuO$_4$ and YBa$_2$Cu$_3$O$_{6+x}$. Experiments were
performed using the chopper spectrometers HET and MARI at the ISIS spallation
source. We have placed our measurements on an absolute intensity scale, this
allows systematic trends to be seen and comparisons with theory to be made. We
find that the insulating S=1/2 antiferromagnetic parent compounds show a
dramatic renormalization of the spin wave intensity. The effect of doping on
the response is to cause broadenings in wave vector and large redistributions
of spectral weight in the frequency spectrum.
