• Participants with TSC (1 month to \< 2 years of age), or DS (1 year to \< 2 years of age), or LGS (1 year to \< 2 years of age) within the specified age range at the time of initial informed consent.

• Parent(s)/legal representative is/are willing and able to give informed consent for participation in the study.

• Parent(s)/legal representative is/are willing and able (in the investigator's opinion) to comply with all study requirements (including accurate electronic participant-reported outcome \[ePRO\] diary completion).

• Participants with TSC must have a diagnosis per the 2012 International Tuberous Sclerosis Complex Consensus Conference. Participants with LGS or DS must have a diagnosis that is consistent with International League Against Epilepsy (ILAE) guidelines and confirmed by the Epilepsy Study Consortium (ESCI).

• Participants who have uncontrolled seizures, and who are currently receiving 1 or more antiseizure medication (ASMs).

• A suitable VEEG, as available in the medical record, within 1 year of Visit 1. When a historical VEEG is not available, and if clinically indicated and appropriate (due to uncertainties or new seizures), a VEEG will be completed and read to confirm diagnosis prior to Visit 3. All VEEGs are to be read at baseline by the investigator and by an independent reviewer.

• Has seizures which are not adequately controlled through their current ASMs, defined as ≥ 1 seizure reported on the seizure diary during the screening/baseline period