The methodological advances in the isolation and culturing of different types of liver cells have shown that the reciprocal interactions between hepatocytes and non-parenchymal liver cells, and between different types of nonparenchymal cells, play an important role in liver functions under both normal and pathological situations. The expression of many intercellular mediators has been shown to be regulated by other molecules released by various types or the same type of liver cell, thus providing a basis for the existence of positive or negative feedback loops. From the teleological point of view, the intercellular signaling networks within the liver act to provide effective support of parenchymal liver cell function. Thus, sinusoidal endothelial cells and Kupffer cells protect hepatocytes from huge amounts of noxious materials arriving in the blood, due to their huge capacity for endocytosis and phagocytosis, and the ability to induce in the liver rather the state of peripheral tolerance than induction of immunity (Knolle and Gerken 2000). Stellate cells in normal liver, apart from their role in the metabolism of retinoids and the regulation of sinusoidal blood flow, control the production and degradation of extracellular matrix components that build the basal framework of the organ structure.