Molecular diagnostics, i.e. the detection and analysis of disease-related changes of DNA or RNA, is becoming ever more important for the diagnosis of bone marrow (BM) neoplasms. In modern BM haematopathology, molecular diagnostics should always be part of an integrated diagnostic approach including clinical information, morphology and immunophenotyping. It is the responsibility of the haematopathologist to interpret the information gathered and to produce a final diagnosis. For this purpose, the practising haematopathologist must be familiar with the various molecular techniques needed and possess an in-depth knowledge of their applications for the diagnosis of BM neoplasms. The first part of this chapter focuses on the most important molecular techniques currently used in everyday diagnostics in the modern haematopathological laboratory. The second part highlights the major molecular and genetic aberrations of diagnostic value across the different haematological disease entities. Ideally the haematopathological laboratory should either be able to perform the relevant tests or be in close cooperation with a laboratory performing them for optimal diagnostics. Such cooperations also include multidisciplinary conferences, where clinicians and haematopathologists meet to discuss the diagnoses of afflicted patients.