Aspects of integrable quantum field theories with boundaries

The purpose of the talk is to give the flavour of a dynamical argument which makes use of boundary states in order to calculate the dependence of certain quantities, such as reflection factors, on the coupling constants introduced at a boundary by integrable boundary conditions. The idea is to consider the boundary state spectrum from two complementary points of view. The first is based on generating the spectrum using bootstrap techniques while the second uses a semi-classical quantization of classical periodic solutions which are confined to a region near the boundary, Comparing the two viewpoints leads to new dynamical information. Rather than attempting a general discussion the talk will concentrate on two examples. The first illustrates the principal features and can be computed exactly-simply a free massive field. The complexities and subtleties of the main ideas are illustrated within the sine-Gordon model.