By the use of a concept due to Crisson, negative resistance is shown to be a phenomenon controlled by either current or voltage but not by both together. Thus, two main classes are found differing in the shape of the volt-ampere characteristic, in the conditions for stability, in the effect of an added positive resistance, and in the effect of an internal time lag. Reliability, good power conversion efficiency, and a low ratio of the unavoidable self-reactance to the negative resistance are mentioned as being desirable characteristics of any negative resistance. A figure of merit for the voltage controlled type is shown to be l/ωCvRv, where Cvis the unavoidable effective shunt capacitance. In the current controlled case Rc/ωLc, is shown to be a merit factor, Lcbeing the effective series inductance. The addition of external positive resistance lowers the figure of merit for the combination. A number of well-known devices are discussed after classifying them in three groups according to principle of operation. The simple group includes such devices as the dynatron and arc which produce negative resistance between two elements. The direct coupled group comprises primarily vacuum tubes having negative transconductance in which negative resistance is produced by a direct connection between the controlling and controlled electrodes. The general properties of the negative transconductance tube as a negative resistance are detailed, including the effect of interelectrode capacitances. The third group, the reverse phase coupled group, is included but not treated in detail.