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.

Experiments show that the response of the temperature in most cases is determined by diffusion. It is in principle possible to determine elements of the matrix of transport coefficients. Interestingly, off-diagonal elements in the transport matrix appear to be important.

In this paper experimental techniques, analysis techniques, basic formulas etc. are briefly reviewed. Experimental results are summarized. The fundamental question whether the fluxes are linear functions of the gradients or not is discussed.

When inducing edge perturbations, often plasma responses are observed which cannot be explained by a local transport model. These so-called ‘non-local’ phenomena have drawn strong attention in the last couple of years, and we will review this class of experiments as well.