The large-scale neuronal networks that underpin normal brain function are disrupted during seizures, which are characterized by a transition to abnormal, hypersynchronous neuronal activity. Many factors can contribute to transitions from interictal to ictal states, and an enduring predisposition to spontaneous, dynamic changes results in recurrent seizures - that is, epilepsy. Unpredictability and the apparent randomness of seizure occurrence seem to be a hallmark of many epilepsies, yet clinicians and patients are aware of periods during which a variety of converging factors may increase the risk of seizures.