  Substitutional impurities in the CuO2 planes of the cuprates allow us to
probe the electronic properties of the host material. The pseudo-gap in the
underdoped regime is unmodified far from the impurities even though Tc is
greatly reduced. The spin polarisation induced by magnetic impurities has an
oscillatory behaviour reflecting the existing AF correlations between the Cu
spins. Its influence on the NMR spectra opens a way to determine the q
dependence of the static spin susceptibility and the T dependence of the AF
correlation length. NMR measurements demonstrate that non-magnetic impurities
such as Zn induce a local moment behaviour on the neighbouring Cu sites. This
magnetism revealed by spin-less sites can be understood on theoretical grounds
in the case of undoped quantum spin systems, while here the carriers greatly
complicate the situation. Susceptibility data show that the magnitude of the
local moment decreases with increasing hole doping. This experimental evidence
directly reflects the influence of AF correlations and the interference between
the carriers and the Cu hole spins in the cuprates. The anomalously large
scattering of the carriers on spinless defects is another indication of the
originality of the electronic properties of the cuprates, which apparently
extends even to the overdoped regime.
