The cell cycle is a tightly coupled series of events that enable cells to grow and proliferate. Cyclin-dependent kinases (CDKs) play crucial roles in the cell cycle by enabling cells to transition between different phases when they are activated. Cell cycle proteins enhance the activity of CDKs, while natural CDK inhibitors (CDKIs) suppress them. The cell cycle continues in cycles under normal conditions, but when conditions change, cells halt or terminate the cell cycle. Tumors are tissues that grow out of control, and the mechanisms of various types of tumors are different; however, almost all tumor cells share several common characteristics, including proliferation, prevention of apoptosis and genomic instability. Cellular division is essential in the progression of cancer. A key characteristic of cancer is the uncontrolled growth of tumor cells, which is due to the erratic behavior of several proteins during the cell cycle. Therefore, cell cycle regulators are considered attractive targets for the treatment of cancer. The present analysis highlights proteins that play a direct role in controlling the tumor cell cycle, such as CDKs, and provides a brief overview of checkpoint kinases. The present review also discusses how cell cycle proteins contribute to cancer and describes some of the antitumor drugs that are being researched.