CDK4/6 inhibition represents a new generation of cancer therapies, targeting CDK4/6 complexes to induce cell cycle arrest in the G1 phase. These inhibitors have been widely used in combination with hormone receptor antagonists for treating ER+/HER2− breast cancer, achieving significant clinical success. Building on this progress, ongoing research explores novel combination therapies and expands the application of CDK4/6 inhibitors to other diseases. However, challenges remain, including variable cellular responses and the rapid development of drug resistance. Recent studies have uncovered new resistance mechanisms and their unexpected effects on cell metabolism, autophagy, and the tumor microenvironment beyond cell cycle arrest. This review provides a comprehensive overview of the mechanisms by which CDK4/6 inhibitors combat cancer and explores their potential for more effective and personalized treatment strategies.