A 'generalised seizure' corresponds to an ictal manifestation whose initial semeiology indicates or is consistent with more than minimal involvement of both cerebral hemispheres. A 'network' is a functionally and automatically connected set of cortical and subcortical brain regions in which activity in any one part affects activity in all others. A 'system' is a group of independent but interrelated elements (networks) comprising a unified whole. A considerable amount of evidence supports the existence of cortical and subcortical networks in the genesis, expression and control of generalised seizures. Animal and human data suggested that the so-called generalised seizures involve selective networks while sparing others. A greater understanding of those systems and networks is thoroughly debated in this book.