This is an international, multicentre, double-blind, controlled, randomized phase II study comparing the efficacy and safety of fulvestrant in combination with palbociclib versus fulvestrant plus placebo in postmenopausal women with HR-positive/HER2-negative metastatic breast cancer who have received ≥5 years of endocrine therapy in the adjuvant setting as treatment for early disease and remained disease free for > 12 months following its completion or have "de novo" metastatic disease. Patients must have at least one lesion (measurable and/or non-measurable) that can be accurately assessed at baseline and is suitable for repeated assessment by CT, MRI or plan x-ray. Patients with bone-only disease must have a lytic or mixed (lytic + blastic) lesion, which has not been previously irradiated and can be accurately assessed by CT/MRI or x-ray. Approximately 190 patients will be randomized 1:1 between the experimental arm (approximately 95 patients treated with fulvestrant plus palbociclib) and the control arm (approximately 95 patients treated with fulvestrant plus placebo).