Inclusion Criteria:

• Males and females between 18 and 65 years of age, inclusive, who were diagnosed with migraines at ≤ 50 years of age, and have a history of chronic migraine for ≥ 12 months before screening.
• During the 28 day screening period, subjects must adequately complete the headache eDiary and must have headaches occurring on ≥ 15 to ≤ 26 days of which at least 8 must be migraine days.
• Headache eDiary was completed on at least 24 of the 28 days prior to randomization.

Exclusion Criteria:

• Confounding and clinically significant pain syndromes (e.g. fibromyalgia, chronic low back pain, complex regional pain syndrome).
• Psychiatric conditions that are uncontrolled and/or untreated, including conditions that are not controlled for a minimum of 6 months prior to screening. Patients with a lifetime history of psychosis, mania, or dementia are excluded.
• Any use of botulinum toxin for migraine or for any other medical/cosmetic reasons requiring injections within 4 months prior to screening and during the screening period.
• History or diagnosis of complicated migraine (ICHD-III beta version, 20134), chronic tension-type headache, hypnic headache, cluster headache, hemicrania continua, new daily persistent headache, or sporadic and familial hemiplegic migraine.
• Receipt of any monoclonal antibody treatment (within or outside a clinical trial) within 6 months before screening.
• Previously dosed with ALD403 or any monoclonal antibody targeting the CGRP pathway.