The features of the enterohepatic circulation of bile acids in mammals are reviewed. Inputs into the circulating bile acids are primary bile acids synthesized from cholesterol in the hepatocyte and secondary bile acids formed by bacterial modification of primary bile acids in the distal intestine. Intestinal conservation of bile acids generates pools of individual bile acids whose relative sizes determine biliary bile acid composition. Efficient hepatic clearance results in low plasma bile acid levels, and virtually no renal excretion. Methods for characterizing the enterohepatic circulation are summarized. Bile acids have numerous physiological functions in the liver, biliary tract, and intestine resulting from their signaling and physicochemical properties.