Genetically engineered crops that produce Bacillus thuringiensis (Bt) toxins kill some key insect pests and can help to reduce reliance on insecticide sprays.  Bt crops have grown on more than 200 million ha worldwide since their commerical introduction in 1996.  This widespread use has raised two pressing questions: 'How quickly will insects evolve resistance to Bt toxins?' and 'Will resistance be delayed by planting refuges of non-Bt crops near Bt crops?'  Here we summarize our recent paper that addresses these questions by analyzing global resistance monitoring data in conjunction with results from computer simulations of the refuge strategy.