  The present paper develops a general theory of quantum group analogs of
symmetric pairs for involutive automorphism of the second kind of symmetrizable
Kac-Moody algebras. The resulting quantum symmetric pairs are right coideal
subalgebras of quantized enveloping algebras. They give rise to triangular
decompositions, including a quantum analog of the Iwasawa decomposition, and
they can be written explicitly in terms of generators and relations. Moreover,
their centers and their specializations are determined. The constructions
follow G. Letzter's theory of quantum symmetric pairs for semisimple Lie
algebras. The main additional ingredient is the classification of involutive
automorphisms of the second kind of symmetrizable Kac-Moody algebras due to Kac
and Wang. The resulting theory comprises various classes of examples which have
previously appeared in the literature, such as q-Onsager algebras and the
twisted q-Yangians introduced by Molev, Ragoucy, and Sorba.
