Cytokines are involved in a number of physiologic and pathophysiologic processes in the liver. These small molecules impact hepatic growth and repair, regulate inflammation, and influence fibrogenesis. Cytokines acting within the liver derive from both resident and recruited cells; the pattern of cytokines produced in a given clinical situation depends upon the inciting stimulus and the background cellular milieu. Experimental animals display inherited differences in cytokine responses to environmental insults. If these differences are operative in human beings, they may account in part for variations in susceptibility among individuals or groups to certain types of liver disease.