Inclusion Criteria:

• * Participant is ≥ 18 years of age at the time of enrollment in the Registry.
• * Participant must have a confirmed diagnosis of AQP4+ NMOSD.
• * At the time of enrollment in the Registry, participants must be receiving treatment with ALXN-C5IT for the purpose of chronic relapse prevention in a manner consistent with the local label. Specifically, they should have received at least 1 dose of eculizumab within 4 weeks prior to enrollment or at least 1 dose of ravulizumab within 12 weeks prior to enrollment.
• * Participants must have both the following historical data available to be enrolled in the Registry: ALXN-C5IT dosing information since initiation and number and types of relapses from 1 year prior to ALXN-C5IT initiation through Registry enrollment.

Exclusion Criteria:

• * Participants currently enrolled in an interventional clinical study for the treatment of AQP4+ NMOSD in which the intervention is a drug.