The evaluation of minimal residual disease (MRD) is a new diagnostic method which is applicable in various malignant disorders. Acute lymphoblastic leukaemia (ALL) is a somewhat ideal disease in this respect because >90% of the patients show individual clonal markers and because several methods for MRD evaluation are already established. Futhermore, it was demonstrated that level and course of MRD are significantly correlated with relapse risk in childhood and in adult ALL. In clinical practice MRD evaluation may serve for several purposes such as follow-up of individual course of disease, identification of new prognostic factors or evaluation of single treatment elements. We discuss these options as well as general considerations for MRD-based risk stratification and treatment options for risk groups. Practical applications are analysed because prospective MRD-based clinical trials have been recently started. Finally, future options for application of MRD evaluation and also limitations and pitfalls of this method are reviewed.