A significant portion of cell scientific literature published is dedicated to describing the cloning, the link to cancer, or the characterization of proteins involved in the progression of the cell cycle. With this abundance of information, the cascading pathways of molecular events that occur in the cell cycle are proving to be exceedingly complicated. Originally, the sole regulator of the fission yeast cells division cycle, cdc2, was thought to also regulate mammalian cell cycles in the same manner. However, mammalian cdc2 has now been joined by seven well-characterized relatives acting at distinct points in the cell cycle. These kinases are activated by larger proteins called cyclins, named with respect to their cyclical expression and degradation. Therefore, the catalytic subunits of these complexes are named cyclin-dependent kinases (cdks). In the event that the cell must stop normal cycling behavior, a number of cdk inhibitors, which have only begun to be characterized, function in inhibiting the kinase ability of cdks, among other nonproliferative acts. The external environment manipulates cellular proliferation and differentiation by stimulating or inhibiting certain signal transduction pathways. However, each component of the cell cycle machinery, as they are the final executors in cell division, has the potential to elicit or to contribute to a neoplastic phenotype. This review focuses on the characterization of each member of the cell cycle protein family and also addresses the potential role each plays in cancer.