Electric currents in solids are usually associated with the presence of electrons which are free to move under the influence of a field. In most cases these free current carriers exist in the charge neutral material. In this article the introduction of excess charge is discussed and it is shown how such charge can carry a current. The use of space-charge conduction as a tool in the study of defects and surfaces is discussed. The simultaneous injection of positive and negative charge at opposite electrodes is also considered. The bearing of space-charge conduction on device technology is reviewed.