6MbAbstractIn this thesis we study 2 dimensional N=(4,4) superconformal field theories and their application to superstring theories. More specifically, we would like to analyze the behaviour of the fields under a certain set of discrete symmetries of a non linear sigma model whose target space is a K3 surface, in particular, the T^4/Z_2 orbifold. A good way to understand this is to compute the Elliptic Genus and some Twining Genera obtained by inserting in the Elliptic Genus one of the…