Crops expressing insecticidal proteins from Bacillus thuringiensis (“Bt crops”) are becoming increasingly popular with farmers as a way of increasing crop yield and reducing insecticide use. However, some fear that widespread cultivation of Bt crops will accelerate the rate at which insects acquire resistance to the toxin. To prevent this, farmers have adopted planting strategies to provide Bt-susceptible insects with stands of nontransgenic plants as “refuges,” which will perpetuate susceptible alleles in the insect population. On page 339, Shelton et al. describe results of a two-year field study in which they released insects with a known frequency of resistance into plots planted with various types of refuges, to investigate the impact of the refuges on the size of the insect population, as well as the frequency of resistance alleles.