An elastic surface resists not only changes in curvature but also tangential stretches and shears.  In classical plate and shell theories, e.g., due to von Karman, the latter two strain measures are approximated infinitesimally.  We motivate our approach via the phenomenon of wrinkling in highly stretched elastomers.  We postulate a novel, physically reasonable class of stored-energy densities, and we prove various existence theorems based on the direct method of the calculus of variations.