  Inelastic neutron scattering experiments in high-$T_c$ cuprates have
evidenced a new magnetic excitation present in the superconducting state. In
particular, recent experiments on single layer Tl$_{2}$Ba$_{2}$CuO$_{6+\delta
}$, performed near optimum doping ($ T_{c} \sim 90$ K), provide evidence of a
sharp magnetic resonant mode below $ T_{c}$, similar to previous reports on the
YBCO and BSCCO bilayer systems. This result supports models that ascribe a key
role to magnetic excitations in the mechanism of superconductivity.
