This chapter reviews the literature on sex differences in olfactory ability and describes the current state of knowledge on this subject. Sex differences in olfactory function have been observed on virtually all olfactory measures examined. These include detection threshold, sensitivity, discrimination, identification, naming, memory and hedonics. The chapter outlines methodological shortcomings in published reports, and offers some hypotheses in order to explain the male/female difference in olfactory function. The consistent behavioural differences between males and females on olfactory psychophysical testing support the argument that it is imperative that 'sex' be taken into account when designing olfactory neuroimaging studies. There is a widely held belief that women are differentially sensitive to odours over the course of the menstrual cycle. The effects of sex on odour processing are not limited to sexually relevant odours, as is evidenced by sex differences in odour detection threshold for sexually irrelevant odorants.