 The study is a randomized, double blind, placebo controlled, Phase 3 clinical trial with the primary objective of demonstrating the superiority of palbociclib in combination with fulvestrant Faslodex over fulvestrant alone in prolonging PFS in women with HR, HER2 negative metastatic breast cancer whose disease has progressed after prior endocrine therapy. The safety between the two treatment arms will also be compared. During study treatment, pre- and perimenopausal women must be receiving therapy with the LHRH agonist goserelin Zoladex or generic. 