This study is a trial comparing the effects of erenumab to placebo in children ages 6-17 years old with migraine 15 days per month or more. Eligible participants will be given either erenumab or placebo, assigned randomly (like the flip of a coin), followed by an open-label phase in which all participants receive erenumab. Erenumab has been FDA-approved for use in adults to decrease the frequency of migraine. Patients may be eligible to participate if they have had migraine for at least one year, and agree to come to CHOP's main hospital for several study visits. Patients whose headache started suddenly (New Daily Persistent Headache), patients whose headache started after a concussion (Post-Traumatic Headache), and patients with severe depression are not eligible for this study.