In the last decade or so, technologies for genome analysis have rapidly been grown and in these days, the whole genome of one species can be sequenced within a few days. As of June 4, 2015, genome sequences for a total of 8311 species are available in the database. Technologies in neuroscience have also been improved by the introduction of molecular biology techniques, for example, using viruses, and the neural circuitry leading to emotion or behavior is being revealed. Thanks to these technology developments, the olfactory research has extensively been progressed, and it is a good timing to summarize the knowledge in this field. In this issue, we review recent progresses in the olfactory research at the levels of molecule, genome, receptor, neural circuitry, hormone, and behavior in various species. The followings are the topics that we will discuss in this special issue: Molecular evolution of olfactory receptor genes (Niimura), Olfactory receptor genotype and perception (Sato), Ectopically expressed olfactory receptors (Kaneko), Olfactory neural circuitry (Ishii), Relationship with hormone and immune systems (Hirasawa), Recognition of predators or different species signals (Tsunoda), Signals involved in parental behavior (Tachikawa), Insect chemosensory receptors and behavior (Morinaga), and Roles of volatiles in plants (Nagashima).