  The discovery of superfluid $^{3}$He in high porosity silica aerogels, and
subsequent experimental and theoretical work, have led to a better general
understanding of quasiparticle scattering from impurities in unconventional
pairing systems. It is immensely helpful for understanding impurity effects in
the case of superfluid $^{3}$He that the structure of its order parameter is
well-established. An overview of impurity effects is presented with emphasis on
those experiments which have a quantitative interpretation in terms of
theoretical models for homogeneous and inhomogeneous scattering. The latter can
account successfully for most experimental results.
