The interleukin 1 (IL-1) family is a group of related cytokines including two agonist proteins (IL-1alpha and IL-1beta), each derived by enzymatic cleavage of precursor proteins (pro-IL-1alpha and pro-IL-1beta), and three forms of an antagonist protein (IL-1ra, icIL-1raI, icIL-1raII). IL-1 plays a key role in the onset and development of the host reaction to invasion, being an important factor in the initiation of the inflammatory response and in the triggering of immune functions. Due to its pleiotropic activity and to the high potency of its inflammatory effects, IL-1 activity is tightly regulated in the body by a complex network of control systems. These include the presence of two types of inhibitors, the receptor antagonist IL-1ra and the second type of IL-1 receptor (IL-1RI), which is a natural scavenger of IL-1. Furthermore, regulation of IL-1 activity is attained by a strict hierarchy of binding affinity of the two receptors (the activating IL-1RI and the inhibitory IL-1RII) for the various members of the IL-1 family. Additional levels of control are represented by the presence of soluble forms of both receptors and of immature pro-IL-1 forms with different characteristics of activity and receptor binding capacity. To clarify the features of reciprocal interaction among ligands and receptors, in the attempt to understand the rules regulating the IL-1 system and its effectiveness, a deep analysis of the relationship between structure and function in the proteins of the IL-1 family becomes of key importance. Information on this line has been provided by several groups mainly with studies of mutagenesis of IL-1alpha, IL-1beta and IL-1ra in parallel with biological assays of activity. In this review, a survey of the available data is provided, in order to construct a hypothetical model of the functional structure of IL-1 proteins as a basis for future therapeutic interventions based on genetic and protein engineering.