The internal electric field in multilayer organic light-emitting diodes (OLEDs) is investigated using a combination of experimental measurement and numerical device modeling. This approach results in a detailed understanding of the functioning of a multilayer OLED. The method is applied to a standard device structure that has received broad attention in literature. From the experimental part, we have demonstrated that the average electric field inside the hole transport layer is larger than or equal to the average field in the emission layer over the entire current range. Device simulations fully clarify the situation, giving an insight into the space charge effects as well as the hole and the electron current distributions in the device. In particular, we find that there is a leakage of unrecombined holes towards the cathode at low voltages. We also find a strong variation of the electric field in the tris(8-hydroxyquinolinato)aluminum layer