Cyclin-dependent kinases 4 and 6 (Cdk4/6) are important components of cell cycle activation and control in early G1 phase. Both enzymes and their regulators, e.g., cyclins, play critical roles in embryogenesis, homeostasis, and cancerogenesis. Cdk4/6 are attractive targets for cancer treatment. Recently, numerous selective small molecule inhibitors of Cdk4/6 have been developed. The potential of Cdk4/6 inhibitors, particularly, pyrido[2,3-d]pyrimidine derivatives, as both anti-cancer drugs and 124I- and 18F-radiolabeled tracers for cancer imaging using positron emission tomography is discussed.