  We review the concept of finite-temperature form factor that was introduced
recently by the author in the context of the Majorana theory.
Finite-temperature form factors can be used to obtain spectral decompositions
of finite-temperature correlation functions in a way that mimics the
form-factor expansion of the zero temperature case. We develop the concept in
the general factorised scattering set-up of integrable quantum field theory,
list certain expected properties and present the full construction in the case
of the massive Majorana theory, including how it can be applied to the
calculation of correlation functions in the quantum Ising model. In particular,
we include the ''twisted construction'', which was not developed before and
which is essential for the application to the quantum Ising model.
