A Study Evaluating Oral Atogepant for the Prevention of Migraine in Japanese Participants With Chronic or Episodic Migraine

3101-303-002 Completers: * Eligible participants who completed Visit 7, and Visit 8 if applicable, of the Study 3101-303-002 without significant protocol deviations and who did not experience an adverse event (AE) that, in the investigator's opinion, may indicate an unacceptable safety risk. De Novo EM Participants: * Age of the participant at the time of migraine onset \< 50 years. * At least a 1-year history of migraine with or without aura consistent with a diagnosis according to the ICHD-3, 2018. * History of 4 to 14 migraine days per month on average in the 3 months prior to Visit -1 in the investigator's judgment. * 4 to 14 migraine days in the 28-day baseline period per eDiary. * Completed at least 20 out of 28 days in the eDiary during baseline period and is able to read, understand, and complete the study questionnaires and eDiary per investigator's judgment.

* Requirement for any medication, diet, or nonpharmacological treatment that is on the list of prohibited concomitant medications or treatments that cannot be discontinued or switched to an allowable alternative medication or treatment. * Participants with an ECG indicating clinically significant abnormalities at Visit -1 (De Novo EM Participants) or Visit 1 (3103-303-002 Completers). * Hypertension as defined by sitting systolic BP \> 160 mm Hg or sitting diastolic BP \> 100 mm Hg at Visit -1 (De Novo EM Participants) or Visit 1 (for all participants) * Significant risk of self-harm based on clinical interview and responses on the Columbia-Suicide Severity Rating Scale (C-SSRS), or of harm to others in the opinion of the investigator. * Any clinically significant hematologic, endocrine, cardiovascular, pulmonary, renal, hepatic, gastrointestinal, or neurologic disease. De Novo EM Participants only: * Difficulty distinguishing migraine headaches from tension-type or other headaches. * Has a history of migraine accompanied by diplopia or decreased level of consciousness or retinal migraine as defined by ICHD-3, 2018. * Has a current diagnosis of chronic migraine, new persistent daily headache, trigeminal autonomic cephalgia (eg, cluster headache), or painful cranial neuropathy as defined by ICHD-3, 2018. * Has \>= 15 headache days per month on average across the 3 months prior to Visit -1 in the investigator's judgment. * Has \>= 15 headache days in the 28-day baseline period per eDiary. * Usage of opioids or barbiturates \> 2 days/month, triptans or ergots \>= 10 days/month, or simple analgesics \>= 15 days/month in the 3 months prior to Visit -1 per investigator's judgment or during the baseline period.