Abstract: The mechanisms of space-charge-limited (SCL) current in solids are discussed. The practical case is taken of a wide band-gap, high-resistivity material containing empty shallow trapping states but in which empty deep trapping states are eliminated by the mechanism of defect compensation described by L ongini and G reen (1956). One-dimensional and one-carrier (electron) current through a plane parallel crystal is considered for the case when one contact is ohmic and one contact is blocking. At small forward voltage, current occurs by the predominant mechanism of carrier diffusion and increases approximately as the exponential of applied voltage; in this range, current is very sensitive to temperature changes. At large forward voltage, current occurs by the predominant mechanism of carrier drift and, after a voltage threshold due to the work-function difference between anode and cathode metals, increases very nearly as the square of applied voltage; this result confirms the simplified analysis of M ott and G urney (1940) and is the solid-state analogue of the three-halves power law for space-charge-limited current in vacuum. In this range current varies as the inverse cube of crystal thickness and is relatively insensitive to temperature changes. Between these two current ranges a smooth transition occurs from a diffusion to a drift mechanism of current and a “virtual cathode” is established in the crystal; there is no evidence for the existence of a negative-resistance region during the transition as predicted by S kinner (1955). Simple and accurate analytic expressions are derived describing forward current-voltage characteristics in the exponential and square-law ranges; they show that, depending mainly on crystal thicknesses, high forward conductance or high forward resistance can be achieved. With a strongly blocking anode, reverse current is always very small and very high rectification ratios can be achieved. For current in the square-law range the Fermi-level is nearly constant through the crystal, except near the cathode and anode contacts. This justifies the distinction made by R ose (1955) between shallow traps, which lie above the Fermi-level and do not affect the form of the current-voltage characteristics, and deep traps, which lie below the Fermi-level and profoundly modify the current-voltage characteristics. The discussion is illustrated with numerical results calculated on the basis of an electron mobility of 1000 cm2/V-sec which is intermediate between the value of 200 cm2/V-sec for cadmium sulphide and 9300 cm2/V-sec for gallium arsenide. In conclusion, some possible applications are considered for space-charge-limited current in fundamental solid-state research.