I will address the problem of computing dynamic correlation functions in Integrable QFT’s at finite temperature and out of equilibrium. The approach is based on the form-factor expansion of the correlation functions. Thanks to the integrability, the form-factors at finite temperature can be effectively bootstrapped, through a procedure generalizing the Smirnov’s bootstrap program for vacuum form factors. The method allows to determine the dynamic correlation functions of strongly interacting systems. The talk is based on the work with A. C. Cubero (JHEP 104 (2019)) and forthcoming publications.