Knowledge about the molecular organization principles of the sense of smell in different species has greatly improved in the last decade (Bargmann 2006; Mombaerts 2004a, 2004b; Rodriguez 2007). It is now well established that in many species, odorant molecules are detected by large families of G-protein-coupled receptors (Buck and Axel 1991), whose molecular sequence and structure may vary across species and phyla, but that essentially implement the same function (Bargmann 2006). Interestingly, the insect olfactory receptors display a unique and unconventional membrane topology in comparison to the mammalian receptors, questioning the existence of a coupling with G-proteins (Benton et al. 2006; Vosshall and Stocker 2007). Nevertheless, understanding how odorant information generated by these large arrays of receptors is interpreted by the brain to produce a great variety of behaviors will be the challenge of the next decade. A few questions, which may appear basic with regard to the complexity of the entire olfactory system, are still not answered. Among these, how olfactory information is encoded in brain networks downstream to receptors, remains poorly understood. In recent years, there have been strong debates on this question and it seems that the answer is not as simple as recording from the neurons of these networks. The ambition of this chapter is not to provide a definitive answer, but to present the most relevant results on this question and put them in perspective, helping the reader to appreciate where the field stands in terms of olfactory coding. Since a certain similarity in the olfactory system organization has been observed across species (Kay and Stopfer 2006), we will endeavor to compare between different animal models. Our focus will primarily be on temporal coding, as temporal dynamics, in our opinion, are currently the main aspect of neuronal activity in the olfactory system that is difficult to integrate in a convincing and unanimously recognized theory of olfactory coding.