Summary For many years, T‐cell acute lymphoblastic leukaemia (T‐ALL) has been considered and treated as a single malignancy, but divergent outcomes in T‐ALL patients receiving uniform treatment protocols encouraged intensive research on the molecular biology of this disease. Recent findings in the field demonstrate that T‐ALL is much more heterogeneous than originally believed and extremely diverse outcomes of patients require refinement of T‐ALL classification, leading to subtype‐specific adjustment of treatment. Many different biological features of T‐ALL blast cells have recently been found to contribute to disease development and patient outcome and their analysis could potentially be introduced into improved diagnostics and classification of the disease. This review focuses on five key issues of T‐ALL biology: chromosome aberrations, gene expression profiles, gene mutations, DNA methylation patterns, and immunoglobulin/T cell receptor (Ig/TCR) gene rearrangements. Additionally, molecular monitoring of minimal residual disease, by far the most reliable independent prognostic factor in T‐ALL, has been highlighted in the context of Ig/TCR gene rearrangements. Translation of this biological information into better prognostic classification and more effective treatment should lead to improvement of outcome in T‐ALL patients.

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