This is a multicenter, double-blind, parallel-group, placebo-controlled, study to assess the efficacy, safety, and PK of ZX008 when used as adjunctive therapy for uncontrolled seizures in pediatric and young adult subjects with Dravet syndrome. After an initial Screening and Baseline charting of seizure frequency, subjects who qualify for the study will be randomized (1:1:1) to receive either ZX008 (0.2 mg/kg/day, 0.8 mg/kg/day; maximum dose: 30 mg/day) or placebo. Randomization will be stratified by age group (< 6 years, ≥6 to 18 years). All subjects will be titrated to their randomized dose over a 14-day Titration Period. Following titration, subjects will continue treatment at their randomly assigned dose over a 12-week Maintenance Period. Subjects exiting the study will undergo a 2-week taper, unless they enroll in a follow-on study. Subjects will be followed for post-study safety monitoring. Parents/caregivers will use a diary daily to record the number/type of seizures, dosing, and use of rescue medication.