In this paper the taxonomy and relative importance of various species within the genus Psoroptes are reviewed with respect to inter- and intra-specific variations in cross infestivity and host specificity, site of infestation and morphological characters. Intra-specific variation within populations of the sheep scab mite, Psoroptes ovis will then be discussed critically with respect to virulence (the relative growth of the scab lesion and mite burdens), epidemiology, susceptibility to the systemic acaricide, ivermectin, and resistance to synthetic pyrethroid and organophosphate acaricides. Finally, it is suggested that the ear mite, P. cuniculi, and the sheep scab mite, P. ovis are variants of the same species. Information was collected from published literature and from the results of unpublished studies carried out at the VLA, Weybridge over the last 10 years.