Resistance is an ongoing concern with the management of Helicoverpa armigera in the Australian cotton industry. In response, resistance management strategies (RMS) are in place to either prevent, or retard further development of, resistance to either chemical insecticides or to the Cry1Ac protein in transgenic plants. While these strategies have been successful at slowing down the rate at which resistance has developed to insecticides, they have neither prevented the ultimate spread of resistance to most field populations nor the evolution of new mechanisms of resistance that make resistance increasingly difficult to manage. RMS are built on knowledge of the genetics of resistance and ecology of insects. Where knowledge is lacking we make assumptions about resistance based on general theories of how new genes evolve in populations. Our work with Bt resistance aims to test these assumptions so that we can refine or validate our knowledge and theories. In this way, we can devise strategies for the cotton industry that work, and do so at minimal cost to the grower.