Inclusion Criteria - De Novo Cohort:  The patient is greater than 22 years of age at time of providing informed consent. The patient is willing and able to provide informed consent and has the ability to answer surveys, questionnaires, and use the diary in English. The patient has received a referral for SC Remodulin via Remunity Pump for Remodulin (treprostinil) Injection or has started SC Remodulin via Remunity Pump for Remodulin (treprostinil) Injection within 10 days prior to providing informed consent. The patient has not been previously treated with a parenteral prostacyclin-class therapy in the past 30 days prior to providing informed consent, except in cases where patient initiates in patient parenteral prostacyclin-class therapy via another infusion pump with orders to start SC Remodulin therapy with the Remunity Pump for Remodulin (treprostinil) Injection upon discharge. The patient has not initiated, discontinued, or changed dose of any other medications within 30 days prior to providing informed consent. Exceptions to this are anticoagulants and/or diuretics.  Inclusion Criteria - Transition Cohort:  The patient is greater than 22 years of age at time of providing informed consent. The patient is willing and able to provide informed consent and has the ability to answer surveys, questionnaires, and use the diary in English. The patient has received a referral for SC Remodulin via Remunity Pump for Remodulin (treprostinil) Injection or has started SC Remodulin via Remunity Pump for Remodulin (treprostinil) Injection within 10 days prior to providing informed consent. The patient had been receiving continuous infusion of SC Remodulin via CADD-MS 3 infusion pump per the Remodulin Prescribing Information for at least 60 days prior to providing informed consent and is at a dose of at least 10 ng/kg/min with no change in dose for at least 21 days prior to providing informed consent. The patient has not initiated, discontinued, or changed dose of any other medications within 30 days prior to providing informed consent. Exceptions to this are anticoagulants and/or diuretics.  Exclusion Criteria:  The patient is pregnant, trying to become pregnant, or lactating. For those in the Transition Cohort: The patient has a history of adverse events resulting from interruptions in their Remodulin (treprostinil) infusions over relatively short periods (such as less than 8 hours) as they may be at higher risk in the event of pump failure, especially at night. The patient is enrolled in, has participated within the last 30 days, or is planning to participate in an interventional study. Note: co-enrollment in other observational studies is permitted. For those in the Transition Cohort: The patient is receiving parenteral treprostinil via IV administration.