AbbVie Upadacitinib is a novel selective Janus kinase (JAK) 1 inhibitor. JAK1 inhibition blocks the signaling of many important pro-inflammatory cytokines, including interleukin (IL)-2, IL-6, IL-7, and IL-15, which are known contributors to inflammatory disorders. Study M14-675 is a Phase 3, multicenter, randomized, double-blind, placebo-controlled study designed to evaluate the efficacy and safety of oral administration of a once daily dose of upadacitinib 45 mg QD compared to placebo as induction therapy for up to 16 weeks in subjects with moderately to severely active UC. At the end of either Week 8 (Part 1) or Week 16 (Part 2), if subjects are eligible, they will be offered an opportunity to participate in Study M14-234 Substudy 3 which is a Phase 3 study designed to evaluate the efficacy and safety of oral administration of upadacitinib 15 mg or 30 mg compared to placebo as maintenance therapy in subjects with moderately to severely active UC who achieved clinical response following induction with upadacitinib in Study M14-675. Please consult the Study M14-234 protocol for further details. Subjects who consent and meet all of the inclusion criteria and none of the exclusion criteria will be enrolled into Study M14-675, which consists of two parts: (Part 1) a randomized, double-blind, placebo-controlled 8-week induction study; and (Part 2) an 8-week Extended Treatment Period for clinical nonresponders from Part 1.