This chapter examines the evidence of olfactory regression over the course of primate evolution with the aim of understanding the human sense of smell in an evolutionary context. It discusses the evolutionary morphology of primate olfaction, including the taxonomic variability of the main and accessory olfactory systems. Mammals possess a number of paranasal sinuses, which emanate from the nasal fossae and excavate the surrounding cranial bones. Sinus development occurs in two stages: primary and secondary pneumatisation. Major evolutionary modifications of the nasal fossae differentiate haplorhines from strepsirrhines and all other mammals. Human olfactory mucosa is typically described as limited to the posterosuperior nasal fossae, covering part of the superior nasal concha and the facing wall of the septum. Based on comparisons of the relative size of olfactory structures, some authors have assigned mammals to different categories of olfactory ability. These categories include highly acute (macrosmatic), diminished (microsmatic) or absent (anosmatic).