The study is a prospective, multi-center, open-label clinical trial. Study's purpose is to characterize the pharmacokinetics and safety of four oral anti-epileptics drugs (levetiracetam, valproic acid [divalproex sodium ER or immediate release formulation if inadequate enrollment}, topiramate, and oxcarbazepine) in a non-randomized sample of obese children and adolescents. The study's duration will be up to eleven days (up to seven days of screening and four days of pharmacokinetic sampling). Eligible participants ages 2 to 18 years will be identified through outpatient clinic schedules and inpatient admissions at each clinic site. Participants receiving at least one of the study drugs per local standard of care will have pharmacokinetic concentrations in plasma drawn according to the specific dosing schedule for each drug. Other study measures include demographics, BMI, waist/hip ratio, medical history, concomitant medication history, documentation of study drug oral intake, adverse effects, and physical examination. The sample size will include 24 participants for each anti-epileptic drug (total 96).