CK2 is a protein serine/threonine kinase which is a highly conserved and ubiquitous protein kinase. It is localized in the cytoplasmic and nuclear compartments, which accords with its multiple functional activities in the cell. Pertinent to this is also the recognition that a large number of putative substrates for this kinase have been identified in various compartments of the cell. New evidence from several laboratories has further reinforced the involvement of CK2 in signal transduction related to many cellular functions, thus underscoring the significance of its functional role in normal and abnormal cell growth and proliferation. This volume provides an overview of the state of knowledge