  The NMR technique is quite essential as it permits atomic scale measurements
in materials. The aim of this article is to present the main parameters
accessible to NMR experiments and to relate them to the electronic properties
of simple solids in which electronic correlations are not dominant. It will be
shown that in metals the Pauli paramagnetism, that is the electronic density of
states is accessible through NMR shift and spin lattice relaxation data. Those
permit as well to determine electronic gaps in some electronic structures, or
in the SC state. The capability to reveal by NMR the incidence of impurities
and disorder is also underlined. A complementary article will be dedicated to
the specific properties of strongly correlated materials which have been
revealed and studied by NMR experiments.
