• To compare the efficacy of CP-690,550 (5 mg BID & 10 mg BID) versus placebo for the reduction in severity of plaque psoriasis after 16 weeks of treatment in subjects with moderate to severe chronic plaque psoriasis who are candidates for systemic therapy or phototherapy. • To evaluate safety and tolerability over 52 weeks of treatment with CP-690,550 (5 mg BID & 10 mg BID) in subjects with moderate to severe chronic plaque psoriasis who are candidates for systemic therapy or phototherapy.