Many species are subdivided into a mosaic of genetically distinct populations, which are separated by narrow hybrid zones. By measuring the frequency of genotypes across such hybrid zones, we can estimate the rate of gene flow, the strength of selection etc. - quantities that would be very hard to measure directly. Currently, we (Nick Barton and Ferran Palero) are working on hybridisation between red and sika deer in Scotland (with Josephine Pemberton and Helen Senn in Edinburgh) and on hybrid zones in Antirrhinum species in France (with Enrico Coen in Norwich).  With Michael Turelli (Univ. of California, Davis), Nick Barton is using theory developed for hybrid zones to model the spread of Wolbachia, a bacterium introduced into mosquito populations to control the transmission of dengue fever.