The fundamental relationships concerning double injection into an insulator have been developed using the free‐carrier densitiesnandpas variables. Particular attention has been paid concerning the limitations imposed by the assumptions used in obtaining the solutions for the final integrals representing the voltage, crystal current, and crystal thickness. Many different forms of then(p) relationships have been discussed, particularly those which may be experimentally obtainable. Finally, these theoretical results have been applied to an actual experimental result reported for injection into anthracene, and the theory satisfactorily reproduces the observed current‐voltage dependence. The additional information obtainable from this analysis is discussed.