Chronic lymphocytic leukaemia is the most frequent haematological cancer in adult patients, and its incidence raises with aging. Diagnosis needs several clinical and biological data, but hemogram together with the morphological and immunophenotypic analysis of the lymphoid cells take the major place. If the diagnosis is performed easily in about 65% of the patients, various clinicobiological entities were reported in the past few years that must be identified, at least because some are of adverse prognosis. Moreover, the other chronic lymphoid neoplasms, corresponding to the various low and intermediate grade non-Hodgkin's lymphomas (mainly of follicular type, marginal zone, mantle cell zone), may disseminate within the blood and the bone marrow. Those circulating lymphoma cells must be identified at diagnosis in order to perform the accurate diagnosis and to avoid an erroneous diagnosis of atypical chronic lymphocytic leukaemia. Up to 90% of lymphoid malignancies are B cell disorders, contrasting with only a few cases of T cell origin: some of those latter cases cannot be neglected however, as they may be observed in Western countries. Most recent classifications (REAL and WHO) defined all hematological malignancies: each entity referred to clinical, morphological, immunological, cytogenetic, and molecular findings. The basis of these classifications is pathophysiological, trying in each disorder to define a normal counterpart to the pathological clone. Reviewing main steps of the immune response in the normal patient, corresponding to those involving B cells, it is possible indeed to localize and demonstrate a function for many of the cells that expand in lymphoid malignancies.