The nasal cavity and the elements it comprises are lined by mucosa. This mucosa contains olfactory sensory neurons, which are organized into four different territories: main olfactory epithelium, septal organ, vomeronasal epithelium and ganglion of Grünemberg. From a morphological point of view, these territories could initially be considered as isolated olfactory subsystems, which - as a whole or independently - have been addressed in the contributions enclosed in this Topic.