[en] Electron cyclotron emission at frequencies for which the plasma is optically thick has been used for a long time as diagnostic to determine the electron temperature in tokamaks. When the resonant layer is optically thin the emission contains information on the plasma density (provided the electron distribution is Maxwellian). However the measurement of the electron density is ampered by wall reflection and the presence of multiple resonant layers in large aspect ratio tokamaks. Here theoretical and experimental investigations of the emission by ''optically thin harmonics'' which has led to improved modelling of ECE for the J.E.T. tokamak and an attempt to measure the electron density in T.F.R. are described