Cell division is an integral part of life of a multicellular organism. The phenomena like development, growth and tissue repair (wound healing) are due to cell proliferation. Cell cycle consists of changes occurring during one cell division to the other. It consists of four phases viz. G1, S, G2 and M. This presentation deals with molecular mechanism of cell cycle regulation in eukaryotes. Loss of control on cell cycle regulation leads to uncontrolled cell proliferation resulting into varied types of cancers.