The term ‘chromaffin’ was originally applied to cells of the adrenal medulla which could be stained with chromaffin salts on account of their high catecholamine content. Release of these catecholamines, often as a result of stress, results in elevated plasma concentrations. The contemporary usage of the term ‘chromaffin’ extends to cells with similar morphological and histochemical properties whether or not they demonstrate a chromaffin reaction. As well as adrenomedullary chromaffin cells, extra-adrenal chromaffin tissue also exists throughout the vertebrates principally in relation to elements of the autonomic nervous system, the cardiovascular system and in the retro-peritoneum. This chapter recounts the development of our knowledge of the distribution and biology of chromaffin systems in vertebrates with particular emphasis on the adrenomedullary cells, the ‘small’ cells of autonomic ganglia and other, dispersed chromaffin elements, with the object of relating their known structural and histochemical properties to their probable functions.