Seizures are caused by paroxysmal discharges from groups of neurons, which arise as a result of excessive excitation or loss of inhibition. The key unit of neurotransmission is the synapse, and the fundamental components of synapses are ion channels. Thus, the cause of seizures boils down to malfunction of ion channels. About one third of seizures are caused by genetic abnormalities, mostly involving ion channels. A quarter or so are caused by structural lesions. Patients with such lesions usually have additional neurological abnormalities. Some of these lesions, such as brain tumors, traumatic brain injury, infections, and perinatal brain lesions, are environmentally acquired. Others, including brain malformations, genetic tumor syndromes, and metabolic disorders are genetic or have a strong genetic component. In about half of seizure disorders, no genetic or structural abnormality is evident. Perhaps many of these cases are caused by genetic or acquired channelopathies that are not yet recognized. In addition to genes and the environment, brain (synapse) development has a strong influence on seizures.