This study is an investigation of the electrical conductivity of two-component developer materials using a measurement cell that reproduces the electric, magnetic, and geometric characteristics of the xerographic development zone. Measurements were made on model developer materials having a range of properties. The measured current depends on whether the developer roll is at rest or rotating. For the static case, the dependencies of twocomponent developer conductivity on electric field, toner concentration, toner and carrier size, and carrier surface roughness were measured. A physical model is proposed, compared to the measurements, and shown to describe the major observations. For the dynamic case in which the roller is turning during the measurement, there are additional current contributions due to contact charging of the developer with the electrodes and displacement currents due to motion of charged toner and carrier particles. A physical model for the voltage dependence is proposed and used to separate the currents.