  Many colloidal systems display very non-Newtonian and solid-like behaviour
when concentrated, a striking feature being the apparition of a yield stress.
After recalling some basics about the interactions between colloidal particles,
I present a few experimental facts commonly observed in these systems: aging
and non-linear rheology. A simple phenomenological model is then introduced, in
which the local state of the system is described by a single scalar parameter,
the fluidity. I proceed with comments on heterogeneous flows in some of these
systems. These notes are not intended to be a comprehensive review, and the
reader is directed to the references for further reading
