Cancers are the most frequent and at the same time the most complicated of somatic genetic diseases. Technical progress in the last fifteen years has enabled to analyse the acquired genetic abnormalities found in the vast majority of cancers. This molecular dissection of cancer has led to an understanding of this disease that can basically be viewed as a rupture of the balance between two class of genes, the oncogenes and the antioncogenes. This article defines the properties of these cancer genes and gives through a few examples an insight into the various mechanisms of cancerogenesis.