  A conjecture is presented for the thermal one-point function of boundary
operators in integrable boundary quantum field theories in terms of form
factors. It is expected to have applications in studying boundary critical
phenomena and boundary flows, which are relevant in the context of condensed
matter and string theory. The conjectured formula is verified by a
low-temperature expansion developed using finite size techniques, which can
also be used to evaluate higher point functions both in the bulk and on the
boundary.
