Paediatric T‐cell acute lymphoblastic leukaemia (T‐ALL) is an aggressive malignancy of thymocytes that accounts for about 15% of ALL cases and for which treatment outcome remains inferior compared to B‐lineage acute leukaemias. In T‐ALL, leukemic transformation of maturating thymocytes is caused by a multistep pathogenesis involving numerous genetic abnormalities that drive normal T‐cells into uncontrolled cell growth and clonal expansion. This review provides an overview of the current knowledge on onco‐ and tumor suppressor genes in T‐ALL and suggests a classification of these genetic defects into type A and type B abnormalities. Type A abnormalities may delineate distinct molecular‐cytogenetic T‐ALL subgroups, whereas type B abnormalities are found in all major T‐ALL subgroups and synergize with these type A mutations during T‐cell pathogenesis.