The 'insulin superfamily' is an ancient category of small, structurally related proteins, such as insulin, insulin-like growth factors (IGF) and relaxin. Insulin-like signaling molecules have also been described in different invertebrates, including nematodes, mollusks, and insects. They initiate an evolutionary conserved signal transduction mechanism by binding to a heterotetrameric, membrane-spanning receptor tyrosine kinase. Recent physiological and genetic studies have revealed that, in different metazoans, the insulin signaling pathway plays a pivotal role in the regulation of a variety of interrelated, fundamental processes, such as metabolism, growth, reproduction and aging.