**1. Introduction**

This chapter provides a cursory description of a well-known phenomenon, namely that a significant percentage of people afflicted with certain prevalent disorders causing degenerative neuropathology, depressive and anxiety disorders, progressive loss of memory and communication function such as Autism Spectrum Disorder (ASD), intellectual challenges, as well as post-traumatic stress disorders present with a range of olfactory deficits. Here, we review our understanding of these deficits and their relation to various clinical manifestations such as neurological and neuropsychiatric diseases and disorders, disorders affecting mood, cognition, communication and memory and finally, olfactory deficits as secondary outcome of therapeutic drugs. At the outset, we will briefly describe the olfactory pathway from olfactory sensory neurons in the nasal epithelium to the olfactory bulb and on to olfactory cortical structures and subcortical structures involved in olfaction such as the amygdala. Then, we shall discuss olfaction in the context of normal age-based decline of physiological functions relating olfactory deficits to the onset of neurodegenerative pathology, decline in cognition, memory, ability to communicate as well as with episodes of depression and anxiety.
