The purpose of this observational study is to learn about the effects of etrasimod as a treatment for adult patients with moderate to severe ulcerative colitis. Patients will be treated according to standard of care and will only be included in the study if etrasimod is the best treatment of choice according to the physician and they have not previously taken etrasimod. All patients will be prescribed etrasimod according to standard of care. Tests and doctor visits will be conducted according to standard of care with the exception of health questionnaires about ulcerative colitis symptoms. These questionnaires will be completed by patients at various timepoints during the study using their mobile phone, tablet, or computer. The study is 52 weeks with 28 days of safety follow up. The effects of etrasimod will be analyzed for each patient comparing their disease activity prior to the start of etrasimod.