The disclosure is directed to programming implantable stimulators to deliver stimulation energy via one or more implantable leads having complex electrode array geometries. A programmer is configured to generate an electrical field model from selected stimulation parameters and patient anatomy data. The electrical field model indicates how the electrical field propagation would occur in the patient during therapy. In addition, the programmer may be configured to generate an activation field model from the electrical field model and a neuron model. The activation field indicates which neurons within the electrical field will be activated during the therapy. Either of these field models may be presented to the user via a user interface that also displays a representation of the lead implanted within the patient. The user interface may allow the user to adjust the stimulation therapy by manipulating displayed field or activation model representations.