Electron density fluctuations have been studied with the multichannel refelectometer at JET. Results from three topics are presented: the location of MHD modes, fluctuations during the sawtooth crash and fluctuations before and after the L-mode to H-mode transition. The non-linear response of the homodyne detection system, used to measure density fluctuations is considered. This can introduce artefacts in the data which should be taken into account in the interpretation of the signals.