AbstractSeveral aspects of negative ions in low pressure discharges are treated. The elementary processes, in which negative ions are produced and destroyed, are summarized. The influence of negative ions on plasma operation is analyzed in terms of transport equations. It is shown that diffusion, electric field, power input, plasma stability and plasma boundary layer are strongly affected by the presence of negative ions. Finally, an overview is given of experimental techniques for the detection of negative ions. Some typical results in low pressure discharges are presented.