Cancer is characterized by the independent growth of cells based on a series of genetic defects. These include changes in proto-oncogenes and tumor suppressor-genes and are based on both pre-existent and acquired alterations in the genome of the cell, caused by exogenous and endogenous factors. The development of cancer is not dependent on a single genetic hit, but requires a series of subsequent alterations. A growing tumour forms its own infrastructure consisting of stroma and blood vessels. Furthermore, the presence of a mononuclear infiltrate indicates an interaction with the immune system. Due to an unbalance between the production and disappearance of cells in a given tissue a growing tumour is born from where cells escape that are capable to penetrate through a basal membrane and that can metastasize to various organs. The increased knowledge regarding the biology of cancer opens new ways for cancer treatment such as gene therapy, immunotherapy and the application of inhibitors of neoangiogenesis.