Substitutional impurities in theCuO2planes 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 thoughTcis greatly reduced. The spin polarization induced by magnetic impurities has an oscillatory behavior 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 theTdependence of the AF correlation length. NMR measurements demonstrate that non-magnetic impurities such as Zn induce a local moment behavior on the neighboring 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. ©1999 American Institute of Physics.