In this talk we investigate finite size effects to matrix elements of local operators in integrable QFT. Examples include the vacuum and excited state expectation values, but we also consider off-diagonal matrix elements. Our approach is based on the infinite volume form factors (which are assumed to be known), and the results apply to arbitrary theories with diagonal scattering. One of the main results is an infinite integral series for the excited state mean values, which is a generalization of the LeClair-Mussardo series originally derived for finite size VEVs or finite temperature 1-point functions. This talk will be a slightly modified and extended version of a similar seminar held in June 2015 in Budapest.