The olfactory system plays a central role for the intra- and interspecific recognition and communication both in vertebrates and invertebrates. In my doctoral thesis, I aimed at gaining insights into the signal transduction cascades in the olfactory receptor neurons (ORNs) of two moth species, Spodoptera littoralis and Manduca sexta. In particular, I focused on the diacylglycerol (DAG)-dependent second messenger pathway, which may be involved in pheromone-independent signal transduction, and on the cyclic nucleotide-dependent second messenger pathways, which are involved in sensitization and adaptation. My dissertation consists of four chapters and three appendices that address several key aspects: 1) Chapter I and II contain a characterization of the DAG-dependent signal transduction cascade in the ORNs of Spodoptera littoralis and Manduca sexta and describe ion channels which are directly gated by DAG, modulated by DAG kinase and Ca2+/Calmodulin, and inhibited by lanthanum and protein kinase C (PKC). Chapter II further deals with the identification of transient receptor potential (TRP)-like ion channels in the brain and antennae of Manduca sexta, because TRP channels are activated by DAG. Accordingly, moth DAG-gated channels most likely represent TRP-like channels. 2) Chapter III and IV focus on the characterization of cyclic nucleotide-dependent second messenger pathways in the ORNs of Manduca sexta that are involved in sensitization and adaptation of the olfactory signal transduction cascades. Since the characterized ion channels are directly gated by cyclic nucleotides, modulated by Ca2+/Calmodulin, and inhibited by lanthanum, they resemble cyclic nucleotide gated (CNG) channels. The subsequent Appendix I finally describes hyperpolarization-activated cyclic nucleotide-modulated (HCN) channels that contribute to the resting potential of ORNs. Thus, there are multiple cyclic nucleotide-dependent channels in moth ORNs. 3) Appendix II contains a characterization of a previously unknown PKC-dependent current, and finally Appendix III deals with the cross-talk between different second messenger pathways in moth ORNs.