  We review interaction effects in chaotic metallic nanoparticles. Their
single-particle Hamiltonian is described by the proper random-matrix ensemble
while the dominant interaction terms are invariants under a change of the
single-particle basis. In the absence of spin-orbit scattering, the non-trivial
invariants consist of a pairing interaction, which leads to superconductivity
in the bulk, and a ferromagnetic exchange interaction. Spin-orbit scattering
breaks spin-rotation invariance and when it is sufficiently strong, the only
dominant nontrivial interaction is the pairing interaction. We discuss how the
magnetic response of discrete energy levels of the nanoparticle (which can be
measured in single-electron tunneling spectroscopy experiments) is affected by
such pairing correlations and how it can provide a signature of pairing
correlations. We also consider the spin susceptibility of the nanoparticle and
discuss how spin-orbit scattering changes the signatures of pairing
correlations in this observable.
