The transcription factor Yin Yang (YY) 1 controls many divergent cellular processes, including cell proliferation and apoptosis. These are key to cancer development, as a consequence of which its expression has been studied in an increasingly wide range of human cancers, including lymphoma, breast, prostate, colon, ovarian, cervical, and brain cancers, osteosarcoma, rhabdomyosarcoma, and leukemia. It is a regulatory transcription factor for a wide range of genes, including genes involved in control of the cell cycle and apoptosis, and it can act either to upregulate or downregulate downstream gene expression, depending on the cellular environment, cofactors, and the gene targeted. Its expression has been associated with development of a malignant phenotype in some human cancers; tumor progression, including metastasis; and survival. However, as data on its prognostic significance has become available for more human cancers, its role in tumor progression has become controversial; there is conflicting data on its association with outcome, with some studies showing a favorable and others an unfavorable association. This is probably because of the many different roles YY1 plays in control of proliferation and apoptosis, one or the other of which may be more prominent in any given tumor. These studies are reviewed to give an overview of the increasingly recognized importance of YY1 in human tumorigenesis.