**Abstract**

Acute lymphoblastic leukemia (ALL), can be defined by a family of genetically heterogeneous lymphoid neoplasms derived from B- and T-lymphoid progenitors. ALL constitutes the most common childhood cancer, due to an overproduction of immature lymphoid hematopoietic cells. Genetic analyzes currently provides important information for classifying patients into prognostic groups, genetic analysis also helps to understand the mechanisms of relapse, pharmacogenetics and the development of new potential therapeutic targets, which should help to further improve the results of leukemia. In fact, the new techniques in molecular cytogenetic permits to identify new cryptic abnormalities, these discoveries have led to the development of new therapeutic protocols. The role of cytogenetic analysis is crucial on ALL patient's management. Karyotyping coupled with FISH analysis identifies recurrent chromosomal abnormalities in ALL, many of these abnormalities have prognostic and treatment impact. This chapter summarizes chromosomal abnormalities that are common and classify ALL according to the World Health Organization (WHO) classifications (2016 revision). We will present the main genetic modifications recently identified as well as the sequence mutations which have helped in the elucidation of the pathogenesis of ALL.

**Keywords:** Acute lymphoblastic leukemia, World Health Organization classification, cytogenetic analysis, FISH analysis
