Novel selective estrogen receptor degraders (SERDs) are a promising therapeutic option under investigation for patients with estrogen receptor (ER)-positive/human epidermal growth factor receptor 2 (HER2)-negative breast cancer. The efficacy of novel SERDs in the treatment of advanced disease has prompted investigation into their use in the early disease setting, to reduce breast cancer recurrence. Here, we describe the design and rationale of the phase III, randomized, open-label CAMBRIA-1 and CAMBRIA-2 studies. CAMBRIA-1 and CAMBRIA-2 are comparing the next-generation oral SERD camizestrant versus standard-of-care endocrine therapy (aromatase inhibitors or tamoxifen) in patients with ER-positive/HER2-negative early breast cancer, who are at intermediate or high risk of disease recurrence. CAMBRIA-1 is comparing 5 years of camizestrant versus endocrine therapy in patients who have already received 2-5 years of standard endocrine therapy, with or without cyclin-dependent kinase 4/6 inhibitors, and are without recurrence. CAMBRIA-2 is comparing 7 years of upfront adjuvant camizestrant versus endocrine therapy, with abemaciclib permitted in both treatment arms for the first 2 years. The primary endpoint for both studies is invasive breast cancer-free survival. Secondary endpoints include invasive disease-free survival, distant recurrence-free survival, overall survival, pharmacokinetics, patient-reported outcomes, safety and tolerability.