The sensory system used for smelling or olfaction is called olfactory system or sense of smell. The overall olfactory system is divided into two types, namely, the main olfactory system and the accessory olfactory system and they perform different functions. The main olfactory system detects airborne substances while the accessory olfactory system detects fluid-phase stimuli. There are various types of olfactory disorders based on the type of malfunction. These disorders include anosmia, agnosia and hyperosmia in the loss of smell, which may be total anosmia, incomplete anosmia (partial anosmia, hyposmia, or microsmia), and distorted anosmia (dysosmia). Agnosia refers to an inability to recognize odors despite having a normally functioning olfactory system. Hyperosmia is a rare condition characterized by an abnormally heightened sense of smell. The olfactory problems can be bilateral or unilateral similar to the case of vision and hearing problems. Olfactory dysfunction can be caused due to age, viral infections, exposure to toxic substances, neurodegenerative diseases, and head trauma. Some of the diverse topics covered in this book address the advanced researches related to olfaction and olfactory disorders. Through case studies and examples, constant effort has been made to make the understanding of the difficult concepts as easy and informative as possible for the readers.