By inducing a small electron temperature perturbation in a plasma in steady state one can in principle determine the conductive and convective components of the electron heat flux, and the associated thermal diffusivity and convection velocity. The same can be done for other plasma parameters, like density or ion temperature.

In this paper experimental and analysis techniques are briefly reviewed. The fundamental question whether the fluxes are linear functions of the gradients or not is discussed. Experimental results are summarized, including so-called `non-local' phenomena.