Acute lymphoblastic leukemia (ALL) is a malignant neoplasm of the lymphocyte precursor cells. Among adults it is a relatively rare neoplasm with a curability rate around 30% at 5 years. Currently, the diagnosis and classification of ALL is a multistep procedure that relies on the simultaneous application of multiple techniques that include: cytomorphology, immunophenotype and cytogenetic assays. Some of them have important clinical implications for both diagnosis and predicting response to specific treatment regimens, while the role of others is still to be defined. Over the years, several prognostic factors have been identified and today a risk stratification at diagnosis and during the follow-up is based on the characteristics of the leukemic cells.