As inflammation is a complex series of events, involving many cell types, it is evident that a great many molecules are encompassed by the term 'proinflammatory cytokines'. The aim of this chapter is to provide an introduction to these molecules and a guide to this complex field. We have chosen to structure this chapter on a functional basis, reviewing the principles of innate (natural) immunity and the cell types most closely involved, and their molecular products. Details are found in other chapters.