This is a multicenter, non-interventional, prospective study. The observation period comprises at least 6 months, from the initiation of ESL add-on therapy in adult patients with partial-onset epilepsy not sufficiently controlled with one AED, until the first visit that occurs between 6 and 9 months of follow-up. The observation period will end after 9 months of follow-up even if the final assessment is not performed.