 This is a Phase 3, multicenter, open-label, 2-part rollover study in subjects with CF who are homozygous or heterozygous for the F508del-CFTR mutation and who participated in Studies NCT02070744, NCT02347657, NCT02516410, NCT02392234, NCT02412111, and NCT02508207. The study is designed to evaluate the safety and efficacy of long term treatment of VX-661 in combination with ivacaftor. 