The cell cycle is tightly regulated on many different levels to ensure properly controlled proliferation. Deregulation of cell cycle regulation is a hallmark of cancer. In this book, many aspects of cell cycle regulation are discussed, which include G1, S, M phase control, ubiquitin-mediated degradation, DNA damage response, mitotic spindle checkpoint, the centrosome cycle, Retinoblastoma protein family, the Myc oncogene, and mouse models for tumor suppressors, cyclin-dependent kinases, and meiosis. These chapters written by experts provide an updated view on how the cell cycle is regulated in vivo and about the involvement of cell cycle regulators in cancer.