The purpose of this real world non-interventional study is to learn about the effects of etrasimod as treatment for 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 choice according to the treating physician. Additionally, patients have to be between 18 and 65 years of age and should not have taken etrasimod in the past. All patients will be prescribed etrasimod according to standard of care. Assessments will be conducted according to standard of care with the exception of health questionnaires which will be completed by the patients online on their own device. The study duration is 52 weeks with 28 days of safety follow-up. Patients will visit their treating physician as they would if they were not enrolled in the study. During the study duration, patients will be asked to complete health questionnaires on a regular basis either on their mobile phone, tablet or computer. The effects of etrasimod will be analyzed for each patient comparing to their disease activity prior to the start of etrasimod.