Proliferating cell nuclear antigen (PCNA) plays an essential role in nucleic acid metabolism as a component of the replication and repair machinery. This toroidal-shaped protein encircles DNA and can slide bi-directionally along the duplex. One of the well-established functions for PCNA is its role as the processivity factor for DNA polymerase δ and ε. PCNA tethers the polymerase catalytic unit to the DNA template for rapid and processive DNA synthesis. In the last several years it has become apparent that PCNA interacts with proteins involved in cell-cycle progression which are not a part of the DNA polymerase apparatus. Some of these interactions have a direct effect on DNA synthesis while the roles of several other interactions are not fully understood. This review summarizes the structural features of PCNA and describes the diverse functions played by the protein in DNA replication and repair as well as its possible role in chromatin assembly and gene transcription. The PCNA interactions with different cellular proteins and the importance of these interactions are also discussed.