Chromosomal aberrations are one of the main characteristics of cancer cells. In the seminar chromosomal aberrations were divided into balanced chromosomal aberrations (which do not lose their genes, but their expression changes) and unbalanced chromosomal aberrations (which result in the loss or gain of genetic material). Analysis of chromosomal aberrations and understanding of their functional consequences led to the identification of new proto-oncogenes and tumour suppressor genes responsible for the development of cancer. This knowledge led to the development of new ideas for specific cancer treatments that exploit the deficiency of these cells, that is attack the mutated genes which inactivates cancer cells. Cancer is represented as a set of various diseases caused by genetic alterations that disrupt the balance of cell proliferation, survival and differentiation, and introduce genetic instability, induction of angiogenesis and invasiveness. Examples in the text describe the most common genetic changes that cause aberrant properties of cancer cells, and how these changes stimulate the development of cancer.