Cell is the fundamental molecule of life and is the smallest unit that can replicate through coordinated processes such as cell growth, DNA replication, distribution of the replicated chromosomes to daughter cells, and cell division. There are various regulatory proteins which have the ability to mediate the progression of the cell through cell cycle. Among these proteins, one such important member are cyclins. These with their catalytic subunits called as Cyclin Dependent Kinases (CDKs) form complexes and are activated at specific stages of the cell cycle. Cyclin D1, an important member of cyclin family plays an important role in controlling various biological processes. This book describes the role of cyclin D1 in health and disease.