Tumours deriving from the endocrine cell system can be organized into two main categories namely: endocrine and neuroendocrine tumours. The endocrine group contains all tumours developing from the follicular cells of the thyroid gland (papillary, follicular and undifferentiated carcinomas) and from the suprarenal cortex (adenomas and adenocarcinomas). Such tumours are as a rule identified and evaluated by routine histological staining and special histochemical techniques are usually not applied. The neuroendocrine tumour group is characterized by its content of intracytoplasmic hormonal secretory granules, which can be visualized at the light microscopical level with techniques such as the Grimelius argyrophil reaction or by chromogranin immunocytochemistry. The neuroendocrine tumours can be further subgrouped into those of neuroectodermal or endodermal (epithelial) origin. The neuroectodermal tumours originate from the suprarenal medulla (phaeochromocytomas and neuroblastomas) and from the paraganglia. The endodermal tumours arise in the pituitary gland, the parathyroid gland, the C-cells of the thyroid gland (medullary thyroid carcinoma), the pancreatic islets (insulomas) or in the diffuse endocrine cell system of luminal organs, chiefly the gastrointestinal tract (carcinoids). The carcinoids include some of presumably neuroectodermal type. These contain a cytoskeleton with intermediate filament of neuronal type (neurofilament), whereas the endodermal tumours usually express cytokeratin filament. Individual tumours within each organ can be distinguished by their specific production of various peptide hormones and biogenic amines. Furthermore, amyloid deposits in the stroma may be of use in tumour diagnosis. Modern tissue sampling techniques are presented here, as also are certain aspects of studies on biological behaviour of neuroendocrine tumours by examination of nuclear DNA and proliferating antigen.