Neuroendocrine tumors (secreting bioamines, peptides, hormones, prostaglandins...) represent rather homogeneous group since they correspond to a benign or malignant proliferation of cells belonging to the diffuse neuroendocrine system. This system might be considered as an extension of the concept (APUD System) proposed by Pearse to account for the common properties of these largely distributed cells. Among neuroendocrine tumors we focused since some years on apudomas and particularly carcinoïd tumors. The determination of their various secretion products in biological fluids allows the early and efficient detection of such tumors exhibiting a very slow growth. Finally the in vitro culture of carcinoïd cells appears to be of great interest in various research fields such as oncology, biochemistry and pharmacology.