Approximately 10 to 20% of all cancers worldwide can be attributed to infection by viruses. Seven human viruses are known for their ability to cause cancer: Epstein-Barr virus, hepatitis B and C viruses, human papillomavirus, human T-Cell lymphotropic virus, Kaposi’s sarcoma human virus, and Merkel cell polyomavirus. These viruses have a common trait of encoding proteins which have growth enhancing activity, cell survival activity, and cell cycle checkpoint bypassing activity. In the context of viral infection, the role of these proteins is mainly to promote cell cycle progression, since viral replication necessitates the cellular machinery. However, the fact that these viral proteins also act in the same fashion as cellular oncogenes allows them to drive abnormal cell proliferation, which can lead to cellular transformation. This review will focus on these virus-encoded proteins. Each of the seven viruses will be briefly introduced, and their viral oncogenes will be presented in regard to their known role in carcinogenesis. Emphasis will be placed on their cellular partners, the pathways that are modulated by those proteins, and the molecular mechanisms behind their ability to drive the development of cancer.