Breast cancer (BC) is the most frequently diagnosed cancer in Australian women. Despite the success of adjuvant endocrine therapy to treat patients with hormone receptor positive (HR+) BC, a subset of patients will develop endocrine resistance. This resistance manifests as metastatic, recurrent disease and patients unfortunately receive temporary or no benefit from alternative endocrine agents. Consequently, there is a need to identify the mechanisms allowing tumor cells to evade endocrine therapy. The cell cycle is commonly de-regulated in endocrine resistant BC at the G1/S checkpoint, including the activity of cyclin-dependent kinases (CDK2 and CDK4/6). These CDKs regulate cell cycle progression, and their pharmacological inhibition is currently receiving attention as attractive anti-cancer treatments. We evaluated the therapeutic potential of a panel of selective and pan-specific CDK2 inhibitors (CVT-313, CYC-065, Dinaciclib and SNS-032) and a selective CDK4/6 inhibitor (Palbociclib). We hypothesized that the addition of CDK inhibitors to current endocrine therapy in both treatment sensitive in vitro and in vivo models would prevent the emergence of treatment resistance. We determined sub-lethal doses of the CDK inhibitors, tracked the emergence of resistance in vitro and in vivo and found that cells use differing mechanisms at the G1/S phase of the cell cycle to evade treatment when continuously exposed to monotherapy or combination therapy for 5 or >120 days. This study was successful in demonstrating that combination therapy in long-term in vitro culture was efficacious as it robustly reduced cell proliferation.