Cytokines comprise a group of small proteins released from cells in order to influence the function of other cells. By binding to highly specific cell-surface receptors, they trigger a vast array of intracellular signalling cascades. Cytokines have been described as interleukins, growth factors, interferons and chemokines. Unlike hormones, which act in a similar way, cytokines are produced by many different types of cell and act on many other types. Most of them are produced only after certain stimuli. The most intense field of cytokine activity is without doubt host defence. The liver resembles a central organ of cytokine activity due to the fact that it hosts hepatocytes, which are highly susceptible to the activity of cytokines in a variety of physiological and pathophysiological processes. Moreover, the non-parenchymal cells of the liver, in particular Kupffer cells (KCs), the resident tissue macrophages of the liver, are able to synthesize a variety of cytokines that may act systemically on any other organ of the body, or in a paracrine manner on hepatocytes and other non-parenchymal liver cells. A classic example of how cytokines act can be observed during the acute phase reaction discussed in this article. The role of cytokines in liver development, acute liver injury, liver regeneration, liver fibrosis and liver metastasis is also discussed.