Why do almost all eukaryotes reproduce sexually? The most plausible explanation for the evolutionary advantage of recombination is that it helps to bring together different favourable alleles, accelerating the rate of adaptation. The challenge is to find the strength of selection for recombination, given that selection acts on a very large number of genes, and to estimate this strength from measurable quantities. With Sarah P. Otto (Vancouver), Nick Barton is analysing a variety of stochastic models of selection on multiple recombining loci.