Olfactory dysfunction is well documented as an early nonmotor manifestation of Parkinson's disease (PD). This chapter outlines the anatomy and physiology of the olfactory system and summarizes the pathological changes in the olfactory system in PD. We review the occurrence of -olfactory dysfunction in parkinsonian syndromes and familial parkinsonism. Different methods to assess olfactory function are presented. Their usefulness in routine clinical situations is limited to special diagnostic situations. However, these methods have provided important insights into the pathophysiology of parkinsonism and can help to identify at-risk groups for future neuroprotective trials. Several lines of evidence now suggest that olfactory disturbance reflects Lewy pathology more closely than it reflects striatonigral dopamine deficiency.