Inclusion Criteria:  Patients aged ≥18 years at the time of riociguat treatment initiation Diagnosis of PAH per National Institute for Health and Care Excellence (NICE) 2018 classification Decision to initiate treatment with riociguat as per investigator's routine treatment practice made prior to enrollment in the study  Initiation of riociguat, as per the FDA-approved US label:  At enrollment OR ≤90 days prior to enrollment, with a documented titration regimen (defined as all documented dose changes including, but not limited to: starting dose and dates and highest tolerated dose and dates) Signed informed consent  Exclusion Criteria:  Previously treated with and discontinued use of riociguat for any reason prior to study enrollment (discontinuation defined as an interruption of therapy ≥30 days)  Participating in any of the following:  Blinded clinical trial Clinical trial involving an unapproved drug Investigational program with interventions outside of routine clinical practice Life expectancy <12 months Contraindicated to receive riociguat per the FDA approved US label Use of nitrates or NO donors in any form Use of PDE5 inhibitors PH associated with idiopathic interstitial pneumonias Unable or unwilling to provide informed consent