  In the first part of the book, we classify the automorphic representations of
{\rm GSp}(2) which are invariant under tensor product with a given quadratic
id\`ele class character, via the lifting of automorphic representations of
twisted endoscopic groups. In the second part of the book, we classify the
admissible representations of {\rm GSp}(2, k), k a p-adic field, which are
invariant under tensor product with a given quadratic character of k^\times.
This classification is given in terms of twisted character identities among the
admissible representations of {\rm GSp}(2, k) and those of its twisted
endoscopic groups. The main tool which we use is the trace formula technique.
