A Study of a Drug to be Used in Addition With Another Drug to Treat Adults With Uncontrolled Partial-onset Seizures

This is a 31-week, multicenter, 2-arm, prospective, open-label, non-randomized, Phase 4 study of eslicarbazepine acetate (ESL) as adjunctive therapy in adult subjects with a diagnosis of epilepsy with POS. Two groups of ESL-naïve subjects will be evaluated. The groups are defined as follows: * Arm 1 (ESL as first add-on): This group will include subjects who have been maintained on a regimen consisting of a stable dose of LEV or LTG for at least 1 month (28 days) prior to screening and who have not used any adjunctive treatment. * Arm 2 (ESL as later add-on): This group will include subjects who have been maintained on a regimen consisting of a stable dose of 1-2 AEDs (excluding oxcarbazepine \[OXC\]) for at least 1 month (28 days) prior to screening and who have used adjunctive treatment in the past. The Arm 1 subjects will allow an assessment of the efficacy and safety of ESL in subjects who are early in the course of their disease and being treated with one of the most common first line AEDs. The subjects in Arm 2 are similar to the subject population in the ESL Phase 3 adjunctive epilepsy studies, treatment-resistant subjects who are later in the course of their disease. The inclusion of these subjects in the present study will provide an assessment of the efficacy and safety of ESL as a later adjunctive therapy in a real world clinical setting. In addition, this study will provide data from both Arm 1 and Arm 2 for several behavioral, mood-related, and QOL-related assessments that were not evaluated in the ESL Phase 3 adjunctive epilepsy program. The study will consist of a Screening Phase of 1 to 2 weeks, followed by a 2-week Titration Phase, a 24-week Maintenance Phase, and a Safety Follow-up/Taper Phase of 4 weeks. The last visit in the Maintenance Phase (Visit 9) is considered the End of Study (EOS) visit