  We determine the quark distributions and structure functions for both
unpolarized and polarized DIS of leptons on nucleons and nuclei. The scalar and
vector mean fields in the nucleus modify the motion of the quarks inside the
nucleons. By taking into account this medium modification, we are able to
reproduce the experimental data on the unpolarized EMC effect, and to make
predictions for the polarized EMC effect. We discuss examples of nuclei where
the polarized EMC effect could be measured. We finally present an extension of
our model to describe fragmentation functions.
