Insects rely on olfaction to locate hosts, avoid enemies, find mates, and more importantly, to adapt to an ever-changing environment. The integration of neuroethology and molecular biology over the last half-century has led to the neural architecture and molecular basis of the peripheral insect olfactory system being better understood. Normally, the olfactory sensory neurons (OSNs), housed in corresponding sensilla, detect odorants and then project their axons into the glomeruli of the antennal lobe. The odorant receptor/odorant receptor co-receptor (OR/ORco) complex on the OSNs’ dendrite plays a key role in odorant recognition. In most cases, insect olfactory systems can be summarized via a classic formula; one class of OSN expresses one member of the OR family, and converges to the same glomerulus in the brain. However, recent advances in research on receptor deorphanization and evolution suggest that this process of olfactory coding is more complex. Not only do multiple organs act as the ‘nostrils’ of insects, but also some non-OR receptor families, such as the gustatory receptors (GRs), ionotropic receptors (IRs), and transient receptor potential (TRP) channels, are involved in olfaction. Some studies have found that air pollutants are interfering with insect olfaction. How insect olfaction evolves and adapts to such anthropogenic factors should become an increasingly important topic of research.