Guidelines for the definition and diagnosis of myelodysplasia were set out by the French-American-British Cooperative group (FAB), and the resulting framework has greatly helped the now very large number of workers in many scientific disciplines who are actively investigating the myelodysplastic syndromes (MDS). Most patients with MDS can be readily classified into clinically relevant subgroups by correlation of clinical findings with the findings from well-prepared peripheral blood and bone marrow specimens. However, there are several areas where the standard morphological features are insensitive, but integration of these parameters with histology and cytogenetic and molecular techniques may help us in understanding this fascinating disease.