In this contribution, several specialists describe the current knowledge on the molecular networks that regulate cell cycle progression, with an emphasis on the G1 phase of the cell cycle. The first part of Regulation of G1 Phase Progression is concerned with the individual molecules that form the network, including cyclins, cyclin-dependent kinases, inhibitors of these kinases and retinoblastoma and p53. The second section describes the signaling cascades by which external factors influence the cell cycle network, including mitogens, the extracellular matrix, nutrients and oxygen radicals. The last section describes the effects of specific external conditions on cell cycle progression and are presented such as serum starvation and subsequent re-addition and stress conditions (heat, osmolarity). The final two chapters describe the relation between cell cycle progression with cell differentiation and with apoptosis.