  After a short introduction on frustrated spin systems, we study in this
chapter several two-dimensional frustrated Ising spin systems which can be
exactly solved by using vertex models. We show that these systems contain most
of the spectacular effects due to the frustration: high ground-state
degeneracy, existence of several phases in the ground-state phase diagram,
multiple phase transitions with increasing temperature, reentrance, disorder
lines, partial disorder at equilibrium. Evidences of such effects in non
solvable models are also shown and discussed.
