In this review the main principles of WHO classification of tumours of haematopoietic and lymphoid tissues are discussed and consequences of introduction of this classification for change of diagnostic and therapeutic standards are outlined. Several disorders are currently diagnosed exclusively based on cytogenetic and molecular analysis, which therefore have become part of routine diagnostic management. Moreover, molecular definition has allowed the introduction in some neoplasms of therapy targeted at molecular change characteristic for a given disease. Such an approach is the most advanced in chronic myelocytic leukaemia, where three targeted drugs (imatinib, dasatinib, and nilotinib) are available, but studies concerning other leukaemias and lymphomas are also very advanced.