Inclusion Criteria:  Male or female participants 18 years of age or older at screening visit Participants with confirmed diagnosis of UC and who are prescribed tofacitinib (Xeljanz®) for moderately to severely active UC as per the Swiss label Participants who are willing and able to comply with all scheduled visits, treatment plan, study interventions, and other study procedures Capable of giving personally signed informed consent  Exclusion Criteria:  Presence of clinical findings suggestive of Crohn's disease Any previous exposure to tofacitinib including participation in the tofacitinib clinical program Co-medication with any other advanced therapies for UC (biologics*, azathioprine, mercaptopurine and methotrexate) or any other JAK inhibitor Any identified contra-indications for use of tofacitinib as per the Swiss label Not owning a handheld digital device compatible with the Sidekick Health App, not willing to have it installed on this device or not capable of using the App Investigator site staff or Pfizer employees directly involved in the conduct of the study, site staff otherwise supervised by the investigator, and their respective family members.