The sense of smell is one of the phylogenetically oldest human senses. Nevertheless the number of publications regarding olfaction is marginal compared with other sensory systems. In recent years, however, there have been enormous advances in understanding the main olfactory processes. These range from the first contact of odorants with receptor cells in the nasal mucosa to the olfactory signal cascade to the processing of olfactory stimuli in the central nervous system. This article focuses on anatomy and physiology of the human sense of smell, which consists mostly of sensory input from two neural systems--the olfactory and trigeminal systems. It considers recent biomolecular experiments and functional neuroimaging studies in humans.