Kindling is an animal model of epilepsy produced by focal electrical stimulation of the brain. This chapter: describes the kindling phenomenon; considers the validity of kindling as an animal model and proposes a hypothesis as to how kindling might contribute to human epileptogenesis; presents a critical review of current insights into the underlying mechanisms; and emphasizes that, if progress is to be made in understanding the mechanisms, the network of brain structures underlying kindling must be elucidated. Recent investigations directly related to the network issue are considered, namely studies demonstrating that a brainstem structure, the substantia nigra (SN), can regulate the kindled seizure threshold. Thus, either microinjection of a GABA receptor agonist or a GABA transaminase inhibitor into SN, but not into nearby sites, elevates kindled-seizure threshold. Likewise, destruction of SN, but not of adjacent structures, is associated with an increase of kindled-seizure threshold. These treatments suppress not only clonic motor seizures, but also complex partial seizures and afterdischarge at the site of stimulation. These findings demonstrate that the SN can regulate the intrinsic neuronal excitability of forebrain structures. A hypothesis is advanced that generation of a complex partial seizure requires activation of neurons in the SN which in turn feed back through polysynaptic connections to influence neurons at the site of seizure origin. This nigral influence on neurons at the site of seizure origin is either a direct excitation or a disinhibition. Thus, the seizure represents reverberatory activity within a network of brain structures which includes the SN. Other investigators have proposed that the centrencephalic system subserved seizure propagation; the relationship of the hypothesis proposed here to these earlier ideas is discussed.