  In this review, I outline some principal theoretical knowledge on the
properties of frustrated systems and thin films. The two points I would like to
emphasize: i) the physics in low dimensions where exact solutions can be
obtained, ii) the physics at phase boundaries where spectacular phenomena can
occur due to competing interactions of the two phases around the boundary. This
competition causes a frustration. I will concentrate my attention to thin films
and phenomena occurring near the boundary of two phases of different
symmetries. The case of two-dimensional (2D) systems is in fact the limiting
case of thin films with a monolayer. Naturally, I will treat this case at the
beginning. After a short introduction on frustrated spin systems, I show
several 2D frustrated Ising spin systems which can be exactly solved by using
vertex models. 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.
Thin films are next presented with different aspects: surface elementary
excitations (surface spin-waves), surface phase transition and criticality.
Several examples are shown and discussed. New results on skyrmions in thin
films and superlattices are also displayed.
