1. A method of determining whether a first iris image matches a second iris image, comprising: extracting from said first and second iris images respective first and second templates, each template comprising a spatial arrangement of template codes; overlaying on said first template a master mask comprising a spatial arrangement of master mask codes, each master mask code being representative of an expected variability or matching reliability within a population of iris images of different individuals of a template code of corresponding spatial location; comparing said overlaid first template with said second template; and determining whether a match exists in dependence upon said comparison, said match indicating that said first and second iris images are images of the same human or animal individual.