This monograph (in two volumes) deals with non scalar variational
problems arising in geometry, as harmonic mappings between Riemannian
manifolds and minimal graphs, and in physics, as stable equilibrium
configuations in nonlinear elasticity or for liquid crystals. The
presentation is selfcontained and accessible to non specialists. Topics
are treated as far as possible in an elementary way, illustrating
results with simple examples; in principle, chapters and even sections
are readable independently of the general context, so that parts can be
easily used for graduate courses. Open questions are often mentioned and
the final section of each chapter discusses references to the literature
and sometimes supplementary results. Finally, a detailed Table of
Contents and an extensive Index are of help to consult this monograph