Remibrutinib in Real-world Clinical Practice - a US Sub-study

Inclusion Criteria: * Patients with a confirmed diagnosis of primary CSU by the treating physician. * Aged at least 18 years on the date of enrolment. * Written informed consent of the patient to participate in the study (according to country specifications) and willingness to complete full follow-up period of 24 months. * Cohort-specific observational inclusion criteria: * Cohort 1: Inadequate control of CSU despite licensed dose of sgH1-AH (no other pre-treatments permitted) and decision (independent of study enrolment) to escalate sgH1-AH treatment. * Cohort 2: Inadequate control of CSU despite licensed dose or escalated sgH1-AH(s) (no other pre-treatment with exception of first generation H1-AH permitted) with decision (independent of study enrolment) to switch to remibrutinib treatment as per local label. * Cohort 3: Any other treatment received in addition to H1-AH, any time during patients' CSU treatment history, with decision (independent of study enrolment) to switch to remibrutinib treatment as per local label. Note, occasional steroid rescue medication is out of scope for cohort definition. If a patient had been on continuous steroids for at least three weeks during treatment history, they will be included in cohort 3. Note: Candidate patients must not have initiated the next escalated treatment step (i.e. up dosed AH for cohort 1, or remibrutinib for cohorts 2 and 3) prior to their enrolment to ensure the baseline visit captures their clinical status before treatment escalation. Exclusion Criteria: * Currently enrolled in a clinical trial or on any experimental treatment. * Patients within the safety follow-up phase of a previous interventional or non-interventional study. * Patients who received remibrutinib as an investigational medical product during a remibrutinib interventional study or MAP/PSDS at any time in the past. * Patients not capable or willing to continuously provide ePRO/eDiary data via electronic means throughout the duration of the study. * Patients who are treated with remibrutinib outside of the local label.