Focal seizures are conceptualized as originating within networks limited to one hemisphere. They may be discretely localized or more widely distributed. Focal seizures may originate in subcortical structures. For each seizure type, ictal onset is consistent from one seizure to another, with preferential propagation patterns that can involve the ipsilateral and/or contralateral hemisphere.

Focal seizures can be described by their semiology (features). The features that occur may reflect the regional networks involved in the seizure origin or propagation, often enabling these areas of the brain to be identified. Some features allow identification of the hemisphere involved, others allow identification of the discrete area of the brain (for example a lobe) that is involved. In this section of, features of focal seizures are presented as well as the collection of features that may allow the seizure onset or network to be lateralized to a hemisphere and/or discretely localized to a lobe or a more localized area of the brain.

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