Innate immunity is the first-line host defense of multicellular organisms that rapidly operates to limit infection upon exposure to microbes. It involves intracellular signaling pathways in the fruit-fly Drosophila and in mammals that show striking similarities. Recent genetic and biochemical data have revealed, in particular, that proteins of the Toll family play a critical role in the immediate response to infection. We review here the recent developments on the structural and functional characterization of this evolutionary ancient and important family of proteins, which can function as cytokine receptors (Toll in Drosophila) or pattern recognition receptors (TLR4 in mammals) and activate similar, albeit non identical signal transduction pathways, in flies and mammals.