  Abstarct: Boundary effects caused by the boundary interactions in various
integrable field theories on a half line are discussed at the classical as well
as the quantum level. Only the so-called ``integrable" boundary interactions
are discussed. They are obtained by the requirement that certain combinations
of the lower members of the infinite set of conserved quantities should be
preserved. Contrary to the naive expectations, some ``integrable" boundary
interactions can drastically change the character of the theory. In some cases,
for example, the sinh-Gordon theory, the theory becomes ill-defined because of
the instability introduced by ``integrable" boundary interactions for a certain
range of the parameter.
