The fish gill is a multifunctional organ responsible for respiration, osmoregulation, acid base balance, nitrogen excretion, and metabolism of circulating hormones. Two or more microcirculatory systems subserve these activities and form one of the most complex vascular networks found in any vertebrate. In this article the vascular anatomy of the teleost gill and the role of gill vessels in mediating physiological function are examined. Vascular corrosion replication techniques have been instrumental in resolving the spatial organization of gill microcirculation. Variations in the replication procedures provide information on the interrelationships between the vascular pathways, factors that govern flow distribution, and the physical characteristics of the vessels themselves. Anatomically, gill vessels are as diverse as their physiological functions. Pillar cells, unique to the fish gill, form the lining of the respiratory vasculature and may have substantial metabolic effects on circulating hormones. The non-respiratory pathways appear to be lined with both typical and unusual endothelial cells, although the fine structure and function(s) of these vessels are largely unknown. To date most of the information on gill vessels has been derived from descriptive morphological studies. Further evaluation of the anatomical and physiological correlations of these tissues is predicated upon the application of histocytochemical, morphometric, and other quantitative methodologies as well as an examination of gills from fish with various evolutionary and environmental backgrounds.