Elevated levels of protein kinase CK2 (formerly casein kinase 2 or II) have long been associated with increased cell growth and proliferation both in normal and cancer cells. The ability of CK2 to also act as a potent suppressor of apoptosis offers an important link to its involvement in cancer since deregulation of both cell proliferation and apoptosis are among the key features of cancer cell biology. Dysregulated CK2 may impact both of these processes in cancer cells. All cancers that have been examined show increased CK2 expression, which may also relate to prognosis. The extensive involvement of CK2 in cancer derives from its impact on diverse molecular pathways controlling cell proliferation and cell death. Downregulation of CK2 by various approaches results in induction of apoptosis in cultured cell and xenograft cancer models suggesting its potential as a therapeutic target.