  We study a class of quantized enveloping algebras, called twisted Yangians,
associated with the symmetric pairs of types B, C, D in Cartan's
classification. These algebras can be regarded as coideal subalgebras of the
extended Yangian for orthogonal or symplectic Lie algebras. They can also be
presented as quotients of a reflection algebra by additional symmetry
relations. We prove an analogue of the Poincare-Birkoff-Witt Theorem, determine
their centres and study also extended reflection algebras.
