Abstract: Comparative oncology is a branch of comparative pathology that is relatively new biomedical discipline. The need for a comparative research into tumors across different groups of living beings has arisen from a relatively old notion that most of multicellular organisms may develop tumors. This apparently very simple fact indicate the possible nature of tumor growth, which can be associated with the basic features of the cells of multicellular organisms such as the division, development, growth and differentiation of cells and tissues. Regarding this, tumor growth is commonly defined as a fundamental disorder in the regulation of cell division, growth, differentiation and cell socialization. In all vertebrates, neoplasia is a disease in which genetically altered cells escape from the normal cell-cycle regulation and monitoring of the immune system. This results in a persistent, expanding or infiltrating growth without control and the architecture of the normal tissue. Concerning the facts that mechanisms of cell sociability control and anti-tumor immune monitoring might be different in different group of animals, especially, in different classes of vertebrates, development of disciplines such as comparative oncology and comparative tumor immunology can be of great scientific importance.