Publisher Summary This chapter provides an overview of the concept of the irritative zone and issues related to the mapping of epileptiform activity by functional magnetic resonance imaging (fMRI). The irritative zone, the area of the brain that generates interictal spikes, is usually in the vicinity of the epileptogenic zone. To localize the sources of electroencephalogram (EEG) potentials, dipole modeling is performed where the contour of the scalp voltage field is used to estimate mathematically the location of a dipole that best explains the field distribution. Several assumptions, however, must be made, such as selecting a suitable skull model and the number, orientation, and patterns of allowable generators. These are necessary to solve the “inverse problem” and arrive at possible solutions. In contrast to spike-triggered fMRI, continuous EEG/fMRI recordings allow for full exploration of the temporal dynamics of activation in addition to spatial localization. For each voxel, an entire time series is available for interrogation in contrast to the discrete number of time points available for comparison using EEG triggered fMRI.